You can find this exercise on page 182 of the book Synergistic Golf.
Exercise Type: BIG BALL
Exercise Category: ABDOMINALS, BALANCE, LOW BACK
Sit on ball feet hip distance apart and knees over feet. With arms to the side and spine erect, lift right foot off floor 10x. Repeat with left leg. For challenge, lift leg and extend straight then bend and return back to floor. Focus on posture and head position.