This exercise uses your bodyweight and your balance to work on your core muscles.
Lie on the stability ball with your lower back curvature pressed against the ball. Bend your knees and pressed firmly the feet against the floor. The upper torso should be hanging off the top of the ball. The arms should either alongside the body, crossed on top of your chest or with the hands behind the back of the head.
With the hips stationary, exhale and flex the waist by contracting the abdominals and curl the shoulders and trunk upward. The lower back should always stay in contact with the ball.