You can find this exercise on page 7 of the book Synergistic Golf.
Exercise Type: BIG BALL
Exercise Category: ABDOMINALS, BALANCE
Lie over big ball with hands on floor and legs squeezed together parallel to the floor and top of ball. Maintain strong plank position and slowly walk hands out away from ball while ball rolls down legs. The farther you go, the more difficult it is to balance. Can you walk out the whole way to your toes? Be aware of back position. Keep it straight and abdominals tight. Walk back in and repeat 5 times.