This crunch uses a Stability Ball. The ball allows you a better range of movement because it adapts better to your spine.
Sit on top of an exercise or stability ball with your feet placed firmly on the floor for support. Position yourself so that your lower back is centered on the middle of the ball. Lie back and bring your hands across your chest, or on either side of your head (over your ears).
Raise your head, chest and shoulders up crunching your abs as you sit upright.
Pause for a moment and then lower yourself to the starting position.
For best results, use a full range of motion in this exercise.