You can find this exercise on page 109 of the book Synergistic Golf.
Exercise Type: TOWEL STRETCH
Exercise Category: ABDOMINALS, FLEXIBILITY, LOW BACK
Stand with feet slightly wider than hips. Hold onto ends of hand towel overhead with straight arms. Inhale leaning upper body to the right pushing hips toward the left feeling stretch around left ribcage. Emphasize the stretch by pulling the towel downward with right arm. Exhale return to standing. Repeat alternating sides 10x with control.