You can find this exercise on page 13 of the book Synergistic Golf.
Exercise Type: BIG BALL
Exercise Category: HIPS, LOW BACK, STRENGTH
Sit on big ball. Walk yourself forward and lean back until only your head and shoulders are on top of the ball. Legs should be hip width apart with knees directly over heels. Squeeze your gluteals and lift your hips as to form a plank or bridge with your body from your knees to your shoulders. Can you balance? Challenge yourself by closing your eyes or walk your feet closer together. Arms should be reaching straight toward the sky.