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There are three main muscles that you will use: (1) a crescent shaped muscle just above the pubic area; (2) The area between the 1st muscle and below the navel; (3) just above the navel to your ribs (the one that hurts when you laugh too hard). Isolate the first muscle group, then the second, and then the third. Once you can isolate and clench these muscles, you'll be on your way to doing the stomach ripple. Work on clenching and releasing them individually and then combine the movements.
Practice making the stomach ripples that cause the back and forth movements. Try isolating or clenching each muscle individually.