The Hatha Yoga Pradipika states that Nauli stimulates the digestive fire, thereby removing toxins, indigestion, and constipation. It is considered a Shat Karma, which is an internal cleansing to aid with excess phlegm, mucus, or fat. The Gheranda Samhita, which predates the Hatha Yoga Pradipika, describes Nauli as such: "With great force move the stomach and intestines from one side to the other." It also claims that it destroys all diseases and increases the bodily fire. In addition, Nauli tones the abdominal muscles and massages the internal organs.
Mastery of the three locks, Mula, Uddiyana and Jalandhara Bandha, is essential to practice Nauli. Mula bandha seals vital energy at the perineum (floor of the pelvis) and Jalandhara closes the current at the glottis (pit of the throat) so that any toxin cleansing heat generated in the torso does not move into the higher centers. Uddiyana bandha begins with a forceful exhalation followed by a sharp sucking up of the intestines and diaphragm into a vacuum created in the thoracic cavity.
Begin with Uddiyana. Stand with your knees bent, feet slightly wider than hip width, and the hands braced against the thighs. Lower the chin to nest in the notch between the collarbones, at the pit of the throat. Exhale forcefully so that the lungs empty quickly and, holding the breath, mimic the action of inhalation. Keep the stomach muscles soft and allow the abdomen to be drawn up, like an elevator, toward the chest. The impulse to inhale from the pelvic floor should push open the thoracic cavity, as if the end of a balloon were held at the perineum and inflated from within. Keep the facial muscles soft, quiet, relaxed, and look toward the torso. Maintain the internal hydraulic lift and hold for several seconds. To release, prolong the chin lock and relax the sides of the chest, loosening the internal vacuum and abdominal lift. Relax the throat and lift the chin slightly before you inhale to avoid gasping. The transition out of Uddiyana Bandha should be gentle and vigilant.
To practice Nauli, assume Uddiyana Bandha. Shift some of your weight over to one side and roll the rectus abdominus (the long muscle that’s often referred to as the "six-pack") along the back waist and toward that side. Continue to roll the abdominal organs in a wave-like action along the inner rear surface of the abdominal wall. Complete one wave by rolling toward the front surface and back to where you began. Do each side several times.
I strongly recommend finding an experienced teacher to learn this "kriya"—cleansing. As potent as it can be to cleanse, the powerful vacuum created in the abdominal and pelvic cavities can also lead to disorders. The Pradipika states, "as taught by his Guru," and Sri Iyengar mentions that "it is not recommended for the average practitioner" and that Uddiyana should not be repeated for more than eight times at a stretch during a 24-hour period. In addition, those with heart disease, hypertension, or ulcers should not attempt it