You can find this exercise on page 75 of the book Synergistic Golf.
Exercise Type: BIG BALL
Exercise Category: ABDOMINALS, LEGS - FEET, STRENGTH
Lie on your back with legs straight in the air. Place big ball between ankles and maintain flat spine while keeping legs as straight as possible toward the sky. Drop the ball into hands right above chest while legs separate and stretch inner thighs. Then, throw ball back up between ankles and squeeze ball. Repeat 30-50x. Increase speed.