You can find this exercise on page 148 of the book Synergistic Golf.
Exercise Type: PRE-ROUND WARMUP
Exercise Category: FLEXIBILITY, SHOULDERS
Stand feet hip distance apart with ends of club in both hands, arms extended straight overhead. Bend both elbows bringing club directly behind head in line with ears. Lower elbows as much as possible and try to straighten arms still holding club behind low back against buttocks. Reverse, bending arms at elbows bringing club back overhead. Do not force stretch. Repeat 5x.