You can find this exercise on page 103 of the book Synergistic Golf.
Exercise Type: PILATES
Exercise Category: ABDOMINALS, CLUB CONTROL - ACCURACY, STRENGTH
Sit tall with legs bent hip distance apart and feet flat on floor. Arms are extended straight over knees parallel with floor. Inhale to prepare. Exhale rolling torso behind tailbone as far as possible while keeping abdominals flat. Keep upper spine flexed forward with lower spine (c-curve) and rotate torso, sweeping same arm down across floor and back. Inhale maintaining abdominal connection as spine rotates back to center and sit tall in starting position. Complete 3 repetitions each side.