You can find this exercise on page 274 of the book Synergistic Golf.
Exercise Type: BIG BALL
Exercise Category: ABDOMINALS, STRENGTH
Lie face up on big ball with feet hip width apart and low back resting on top of ball. Body should be parallel to ground. Place hands under base of head for support and contract abdominals bringing upper body closer to sky in crunch motion. Keep abdominals and gluteals engaged at all times. Repeat 3 sets of 10-15 reps.