Position
Lie sideways on a Resista-Ball with your trunk laterally flexed over the ball. Keep a natural arch in your back. (If you don�t have a Resista-Ball, you can use a large pad or firm pillow.) Flex your knees and spread your legs apart as far as needed to stabilize yourself. Begin with tension on your internal obliques with your arms overhead, crossed over your chest, or by your side.

Technique
Contract your internal obliques and begin pulling the side of your rib cage toward the side of your pelvis. Continue until your internal obliques are fully contracted, keeping a natural arch in your lower back. Hold, contracting your internal obliques and breathing out any remaining air. Slowly lower your trunk to the starting position and repeat.