Nauli is one of the six Hatha Yoga Kriyas.  

For an exercise that only takes a few minutes each day to practice yet brings such enormous benefits, Nauli could be considered as the door to a miraculous inner fountain of health and vitality. To find this door, to master the practice, may take days, many weeks, months, or even years but is well worth the persistence and perseverance. Once discovered you will come to understand the great advantages gained and will find it hard not to make this practice part of your daily life.

How do you practice Nauli?

Before attempting Nauli the practices of Agnisar Kriya and Uddiyana Bandha should be mastered. It is best to learn these practices from a qualified Yoga teacher.

Nauli is the churning of the stomach muscles while creating a suction effect in the abdominal cavity. Moving the muscles at the centre of the abdomen (Rectus abdominus) in a circular motion strengthening the abdominal muscles and massaging the intestines and organs of the lower abdomen including the nerves, intestines, reproductive, urinary and excretory organs.

The first step in Nauli is to learn to isolate the rectus abdominus muscle while creating suction in the abdominal cavity. Once this has been perfected, the rectus abdominus muscle can be rolled around in a circular motion.

Nauli helps to balance the adrenal component of the endocrine. It generates heat in the body and dries up excessive mucous. It increases appetite and digestion, and causes happiness. It regulates blood pressure and has a preventative effect against diabetes.
Helpful for heartburn, skin diseases (acne), constipation, indigestion, nervous diarrhoea, acidity, flatulence, depression, hormonal imbalances, sexual and urinary disorders, diabetes, lack of energy and emotional disturbances.

Nauli is a shining gem in the treasure trove of Yoga practices. 

Nauli is one of the best exercises for our health, due to the stimulation and regulating effect upon the entire digestive system. Many illnesses have their origin in the digestive system: headache, skin disease, sometimes also Cancer. Toxic substances and waste products that have not been excreted in a timely manner, are stored in the body – this is the cause of these misfortunes.
Nauli stimulates and purifies the Manipur Chakra, the storehouse of prana and centre of vitality. It helps to increase mental clarity and power by harmonising the energy flows in the body.

Caution: Practice on an empty stomach. Do not practice during pregnancy, menstruation, or if kidney or gallstones are present.

References:
1. Paramhans Swami Maheshwarananda, 2000. Yoga in Daily Life - The System. Ibera Verlag, Vienna.
2. Swami Satyananda Saraswati, 1996. Asana Pranayama Mudra Bandha. Enlarged and revised edition. Bihar Yoga Bharati.
3. Pancham Sinh, 1979. The Hatha Yoga Pradipika. Sri Satguru Publications, Delhi.