The stability ball pass works your core.
Stability Ball Pass Form Cue
Lie on the floor and hold the ball between your feet, legs straight up in the air. Contract your abs and reach for the ball with your hands, passing it from feet to hands. Lower your legs and upper body to the floor, arms overhead. Make sure your low back stays in contact with the ground. Lift back up and pass the ball back to your feet. Repeat. Make sure to keep breathing the entire time, and only lower your legs to the point where you can keep your core stabilized and pulled in.