Nauli in Padmasama demonstrated by Tamara. Nauli is considerably more difficult to perform in sitting positions than in standing positions.
While nauli is primarily a cleansing technique, it’s use within asana practice cannot be overlooked. While many Western yoga disciplines downplay, or even totally ignore nauli, advanced asana practitioners inevitably end up using nauli’s muscular actions within their practice anyway. For example, many yoga postures where one of the instructions is to “move the abdomen to the left [or right],” in order to align the chest with the leg, requires use of the same abdominal muscles used in the right and left side of nauli engagement.
Uddiyana Bandha, the precursor to nauli, is used extensively in asana practice.
In Sri K. Pattabhi Jois’ definitive treatise on the Ashtanga yoga system, Yoga Mala, he mentions the use of nauli in his description of the posture kukkutasana . He writes:
“When in the state of this asana, one should do rechaka [exhalation] and puraka [inhalation] deeply while keeping the chest, waist, and back completely straight. Then, with the heels pressing on either side of the navel, and keeping the head lifted, one should do uddiyana bandha and nauli.”
In this video, Lino Miele demonstrates nauli in kukkutasana:
Other postures also lend themselves to nauli. Adho mukha svanasana (downward facing dog) is another posture where nauli can be performed to bring greater awareness to your core.
In this video from 1938, Tirumalai Krishnamacharya, primary teacher of Pattabhi Jois and B.K.S. Iyengar (among others), is shown doing central nauli in virasana (time index 4:13):
Jois, Sri K. Pattabhi. “Yoga Mala”. North Point Press, 1999. Page. 93.
Yogini Simone Merke Nóbreg demonstrates nauli. She first achieves uddiyana bandha at time index :15, then central nauli at :23. Later, she does uddiyana bandha and starts nauli at time index 1:00.
Some say that the Sanskrit word nauli means “to churn” . B.K.S. Iyengar says that “nau” means “boat” and “li” means “to cling to, lie on, or cover.” He notes that the “pitching of a boat on a stormy sea conveys some idea of the process of nauli.”  Other names for nauli are nauli kriya, lauliki, naulika, nyoli, and chalani. 
The earliest known reference to nauli is in The Hatha Yoga Pradipika, a 15th century yoga text written by an Indian yogi named Swatmarama . The book directs the yogi to (translated from the original Sanskrit):
“Lower the shoulders. Revolve the stomach left and right with the speed of a strong whirlpool. This is called Nauli by the masters. This Nauli is the crown of Hatha practices. It kindles a weak gastric fire, restores the digestion, always brings happiness, and dries up all defects and diseases.”
The Gheranda Samhita, a (probably) 18th century Sanskrit work, provides a shorter description of the technique . Translated from the original Sanskrit:
“Rotate the stomach quickly on both sides. This gets rid of all diseases and the bodily fire increases.”
Nauli is described or mentioned in many modern (20th Century) yoga texts, as well as various journals and magazines. This Google Book search returns around 5000 English language books and magazine mentioning the word nauli – there are a fair number of unusual results (duplicates, references to a person named Nauli, etc), hence the estimate of 5000 out of about 7100 returned results): Google Book Search.
Here is an annotated list of some references (in no particular order):
The Hatha Yoga Pradipika. Translated by Brian Dana Akers. YogaVidya.com, 2002. ISBN: 0971646619.
A recent translation of this classic work with attention given to the primary source material. Required reading.
The Gheranda Samhita. Translated by James Mallinson. YogaVidya.com, 2004. ISBN: 0971646635.
Another modern translation from the YogaVidya catalogue. Refined and valuable.
Light On Yoga. B.K.S. Iyengar. Published by Schocken Books, revised edition, 1995.
Perhaps the definitive modern asana reference. Iyengar describes uddiyana bandha and nauli in the section “Bandha and Kriya,” pages 425-428.
Hathatatvakaumudi. Translated by Dr. M.L. Gharote. Published by The Lonavla Yoga Institute, 2007. ISBN: 819016175X.
A treatise on hatha yoga written by an ayurvedic practitioner named Sundaradeva, probably in the 1700’s. Hathatatvakaumudi means “light on the principles of hatha yoga”. Nauli is described on pages 106-108.
Hatharatnavali. Translated by Dr. M.L. Gharote. Published by The Lonavla Yoga Institute, 2002. ISBN: 8190117696.
Yet another treatise on hatha yoga written by a scholar named Srinivasa, probably in the late 1600’s. Two types of nauli are mentioned (internal and external) though it’s not clear what the difference is between the two. Page 16.
Satkarmasangrahah. Translated by Dr. R.G. Harshe. Published by Yoga-Mimamsa Prakasana, Kaivalyadhama, 1970. Amazon.com ASIN: B0006CNQF8.
Written by Cidghananandanatha, this short text mentions three types of nauli: Bahya nauli, Nalanauli, and Antranauli. Pages 39-40.
Shadow Yoga, Chaya Yoga. Shandor Remete. Published by Shadow Yoga, 2006. ISBN: 0977539008. [Also, revised edition, 2010. ISBN: 1556438761].
Remete offers a short chapter on nauli which he calls ” …the bridging point between asana and pranayama…”
Hatha Yoga. Theos Bernard. Publisher: Essence of Health, South Africa, 2001. ISBN: 0958446016.
Originally written as his Ph.d dissertation for Columbia University in 1943, this classic work discusses uddiyana bandha and nauli with numerous footnotes.
Advanced Yoga Practices: Easy Lessons for Ecstatic Living. Yogani. AYP Publishing, 2004. ISBN: 0976465507.
One of the better modern texts that teaches uddiyana and nauli. Nauli is discussed on pages 208-211.
Heaven Lies Within Us. Theos Bernard. Publisher: Essence of Health, South Africa, 2002. ISBN: 0958446113.
Bernard’s autobiographical account of learning yoga. Originally published in 1939. He describes learning nauli as well as other yogic techniques.
The Yoga Tradition. Georg Feuerstein. Hohm Press, 2008, 3rd edition. ISBN: 1890772186
Feuerstein offers barely 1/5 of a page to nauli, but he provides hundreds of pages of history and background material on the various cultures that spawned the yogic sciences.
Yoga in modern India: the body between science and philosophy. Joseph S. Alter. Princeton University Press, 2004. ISBN: 0691118744.
This book looks at, among other things, some of the medical research about nauli from the early to mid-20th century.
 Yogani, “Advanced Yoga Practices: Easy Lessons for Ecstatic Living”. AYP Publishing, 2004. ISBN: 0976465507. Page 494.
 Remete, Shandor. “Shadow Yoga, Chaya Yoga”. Published by Shadow Yoga, 2006. ISBN: 0977539008. Pages 68-69.
 “The Hatha Yoga Pradipika”. Translated by Brian Dana Akers. YogaVidya.com, 2002. ISBN: 0971646619. Page 41.
 “The Gheranda Samhita”. Translated by James Mallinson. YogaVidya.com, 2004. ISBN: 0971646635. Page 13.
 Daniélou, Alain. “Yoga: Mastering the Secrets of Matter and the Universe”. Published by Inner Traditions, 1991. ISBN: 0892813016. Page 64.
 Iyengar, B.K.S., “Light On Yoga,” Published by Schocken Books, revised edition, 1995. ISBN: 0805210318. Page 427.
Nauli is considered an advanced yoga practice. In this case, “advanced” means there is some risk of injury if nauli is not performed properly. Shandor Remete says that :
“The practice of nauli cannot be learned from books. One needs to work with a teacher who has mastered it and clearly understands its function.”
Of course, the task of finding, and recognizing, such a teacher is left as an exercise for the student.
B.K.S. Iyengar, in his book Light On Yoga, says about nauli :
“Care should be observed in its performance, otherwise the process leads to numerous diseases. It is not, therefore, recommended for the average practitioner. First master uddiyana bandha before attempting nauli….”
Despite Iyengar’s rather ominous warning, he provides no details as to the “diseases” caused by improper performance of nauli.
In a modern textbook, Yogani points out that :
“…if nauli is practiced over a period of time without the benefit of the purification practices of meditation and spinal breathing, it could lead to energy imbalances in the body.”
While it would be preferable to learn nauli from an experienced practitioner, it is not always possible to find one. People can, and do, learn nauli on their own. If one works slowly and with careful attention to the breath, as they would with any new yoga technique or asana, problems should be avoided. Pregnant women and people with heart disease or gastrointestinal illnesses should seek professional counsel before performing nauli. Practicing nauli should never cause physical pain of any kind although there may be some physical discomfort when first learning uddiyana bandha contractions.
Learning nauli typically happens in stages:
Stage 1: learn uddiyana bandha. The student learns to activate and maintain uddiyana bandha. The student commences a regular practice of uddiyana bandha contractions (known as agnisara dhauti, or simply agnisara ).
Stage 2: attempt central nauli. After practicing Stage 1 for some time (weeks or months, most likely), the student occasionally attempts central nauli. Eventually, central nauli is attained.
Stage 3: attempt right and left nauli. After central nauli is attained, the student, continuing their regular uddiyana bandha contractions and central nauli work, attempts right and left nauli.
Stage 4: nauli. Once right and left nauli are attained separately, the student can attempt to roll the abdominal muscles from right to left, and left to right.
Preparation: It’s very important when attempting any of these exercises to do so on an empty stomach. Five hours after eating is a reasonable minimum, but practicing first thing in the morning is a better idea. Also, evacuate your bowels before starting. 
1) Stand with the feet a bit more than hip width apart.
2) Bend the knees slightly and place your hands on top of the front thighs above the knee.
3) Exhale all of your breath out. Some instructions say to “quickly” or “forcibly” exhale, but exhaling in a relaxed fashion works fine . Make sure your abdominals are relaxed as possible once all of your breathe has been exhaled.
4) With your breathe fully exhaled, raise the rib cage and then concave the area below the navel upward and inward back toward the spine. Draw the lower belly in and up toward the spine allowing the lower back to gently round while the pelvis tucks under a bit. Hold this contraction for a few seconds, or as long as comfortable.
5) Release the contraction and then inhale through the nose slowly. If you gasp for air, you held the contraction too long.
Uddiyana Bandha Contractions
Uddiyana bandha contractions are simply the act of going in and out of uddiyana bandha repeatedly (also known as agnisara dhauti, or simply agnisara ). Follow the instructions above for Uddiyana Bandha. Once you’ve achieved the contraction in step #4, release the contraction without inhaling. Then re-establish the contraction (still without inhaling). That’s one contraction. Do as many contractions as you can manage comfortably. Then proceed to step #5 above.
Initially, you may find that you get out of breath quickly. Slowly build your daily practice until you can do 5-10 contractions without inhaling. You may eventually be able to contract and release 10 or more times without inhaling.
Do a few sets of uddiyana bandha contractions to warm-up your abdominal area. Follow steps #1-4 above to achieve an uddiyana bandha contraction. Then try to relax the middle of your abdominal muscles while keeping the side abdominal muscles engaged. Sometimes pressing a bit more firmly on your thighs with your hands helps. Don’t be discouraged if it doesn’t come easily.
Right and left nauli
Do central nauli a few times to warm-up your abdominal area. Then obtain uddiyana bandha and relax the left side abdominal muscles while keeping the right side muscles engaged. Sometimes putting more weight in the right side arm and leg helps. Now contract the left side muscles while keeping the right side relaxed. Again, putting more weight in the left side arm and leg can help.
Practice right and left side nauli a few times. Then try to move directly from contractions on one side to contractions on the other side. After a while, you will find that you can create a rolling motion from one side to the other and back. This is nauli.
Regular nauli practice: Once a regular nauli practice is attained, a typical session involves 500 to 1000 revolutions and requires from 15-25 minutes (depending on your speed), first practicing left to right, and then right to left . The number of revolutions per exhale may vary from just a few to as many as 25 (or possibly more) but the quality of the rotations should never be sacrificed for quantity . The speed of rotations is not important. It can be done either slowly or quickly and the same benefits will be derived.
Helpful videos and descriptions.
This video shows a good view of the standing posture for nauli. Also, if you just watch the first 30 seconds, it shows the person doing uddiyana bandha (but not the release).
Teacher Adrian Cox demonstrates and discusses nauli in this video.
Teacher Larry Terkel demonstrates and discusses nauli in this video.
The AYP site has a rather long explanation of doing uddiyana and nauli.
The Yoga Age site describes uddiyana and nauli with some pictures.
 Iyengar, B.K.S., “Light On Yoga,” Published by Schocken Books, revised edition, 1995. ISBN: 0805210318. Page 427.
 Remete, Shandor. “Shadow Yoga, Chaya Yoga”. Published by Shadow Yoga, 2006. ISBN: 0977539008. Page 68.
 Coulter, David. “Anatomy of Hatha Yoga”. Published by Motilal Banarsidass, 2004. ISBN: 8120819764. Pages 195-206.
 Bernard, Theos. “Hatha Yoga”. Publisher: Essence of Health, South Africa, 2001. ISBN: 0958446016. Page 44.
 Practice note: Uddiyana bandha and nauli are performed while in the state of bahya kumbhaka, which means that you maintain a state where your lungs are emptied of air as much as possible.
 Yogani. “Advanced Yoga Practices: Easy Lessons for Ecstatic Living”. AYP Publishing, 2004. ISBN: 0976465507. Page 210.
 Lysebeth, Andre Van. “Yoga Self-Taught”. Publisher: Weiser Books, revised edition. 1999. ISBN: 1578631270. Page 231. Repeated uddiyana contractions in a standing position are also known as agnisara dhauti, or simply agnisara