You can find this exercise on page 161 of the book Synergistic Golf.
Exercise Type: BIG BALL
Exercise Category: BALANCE, RHYTHM - TIMING
Lie face down on ball resting on crossed arms under chest. Legs remain straight and slightly wider than hips. As one unit, turn entire body to the right, which places left side on ball. Keep rotating to back side and switch left leg under right leg to maintain balance. Reverse rotation back to chest down and repeat process to the left. Repeat 5 times each side.