Nauli is a purificatory process of eliminating the impurities that have gathered in the body. You can practice this process only if you have impurities collected in the body, otherwise there is no need to practice it everyday. This process should be performed in the standing position. Stoop forward slightly and turn the belly swiftly from left to right. Rotate the belly swiftly from left to right and vice versa.
When the abdominal wall becomes concave, the front abdominal walls are contracted and the other muscles of the abdominal walls are relaxed. The abdominal wall consists of three sets of abdominal muscles called abdominal recti which are vertical and run from below the navel till the chest bone, the transverse abdominal muscles running across the abdominal wall and the internal and external oblique muscles. Of these, only the central vertical muscles are to be contracted. This contraction isolates them from the other muscles of the abdominal wall. They stand out like the column in the midst of the concavity of the abdomen. This is caused by making a thrust from behind at the lower end, located at about two inches from the navel. Regular practice makes this task easier.
When you contract the front abdominal muscles in the same way; as you contract the muscles of the anus while defecating at the time of constipation; is called Madhya-nauli or nauli-madhyana. Hold this isolation for a few seconds while the breath is held out. Nauli-chalana involves the isolation of only one of the two abdominal recti at a time, moving it to an end and then to the center, while repeating the same process with the other recti. Nauli is the foremost amongst all the shuddhikriyas or cleansing processes. It affects practically all the functions of the body. Periodic repletion of this activity will ensure that your internals will remain clean and free of any impurities.