Nauli kriya is the isolation, contraction and then rolling engagement and disengagement of the rectus abdominus muscles.
It is composed of three stages:
Madhyama nauli: where both the rectus abdominus muscles are contracted centrally. (L)
Vama nauli: where only the left rectus abdominus muscle is contracted. (C)
Dakshina nauli: where only the right rectus abdominus muscle is contracted. (R)
Once all three are mastered you can begin to work on rotating the muscles, first moving anticlockwise (from left to right), and then clockwise (from right to left). This kriya is best practiced on an empty stomach…and must always conclude moving from right to left to encourage proper function of the gut.