You can find this exercise on page 203 of the book Synergistic Golf.
Exercise Type: PILATES
Exercise Category: ABDOMINALS, BALANCE, FLEXIBILITY, STRENGTH
Balance in seated position with weight behind tailbone and back slightly rounded engaging abdominals tight toward spine. Legs extend in air in a V position wig hands placed behind knees. Inhale by deepening the C curve of the spine allowing entire spine to flex and roll backward without touching head to floor. Maintaining shape, exhale rolling forward and return to starting position. Focus will be placed on consistent abdominal engagement throughout entire movement. Do 10 X.