You can find this exercise on page 129 of the book Synergistic Golf.
Exercise Type: BIG BALL
Exercise Category: ARMS - HANDS, CHEST, STRENGTH
Place hands directly on top of ball shoulder width apart with fingertips aiming downward and thumbs forward. As you gain balance and control, start to put more weight on ball as you walk your feet away from you. Once a plank is achieved, as a challenge, lower your entire body down to the ball in push up fashion, and push back up to straight arms.