Position
Lie on the ball with your back slightly extended, your shoulders higher than the hips, and your sacrum pressed firmly against the ball. Bend your knees to form a 90-degree angle between your upper and lower legs. Spread your legs, placing your knees just outside shoulder width. Begin with your arms lightly supporting your head or crossed over your chest.

Technique
Contract your abdominals, and pull your rib cage toward your pelvis. Try to move only one vertebra at a time. Pull your rib cage toward the pelvis until your abdominals are fully contracted, keeping the sacrum pressed against the ball. Hold, contracting your abdominals and breathing out any remaining air. Slowly lower your trunk one vertebra at a time back to its original starting position, never releasing tension from your abdominals. Repeat.