Position
Lie on your back with your feet resting on a chair, bench, etc., your knees together, and your hips and knees flexed. Rotate your hips and pelvis to one side while keeping your trunk and upper body straight. Place one hand across your side that is facing up, with your other hand behind and lightly supporting your head. Begin with your shoulders slightly off the floor and tension on your abdominals.

Technique
Contract your abdominals and obliques. Begin pulling your rib cage up and over, toward your pelvis. Continue to pull your rib cage toward your pelvis until your abdominals are fully contracted and your trunk and upper body are aligned straight with the angle of your pelvis. Hold, contracting your abdominals and external obliques, and breathe out. Slowly lower your trunk back and down to the starting position.