You can find this exercise on page 238 of the book Synergistic Golf.
Exercise Type: HEAVY BALL
Exercise Category: BALANCE, CHEST, POSTURE, SHOULDERS, STRENGTH
Place both hands directly under shoulders in pushup position and place feet on top of ball. Feet may be pointed with tops of feet resting on ball or flexed balanced on toes. Maintain regular breathing engaging chest and entire midsection. Goal is to be able to hold position for 30 seconds. Extra challenge: lift one foot slightly off ball.