You can find this exercise on page 286 of the book Synergistic Golf.
Exercise Type: YOGA-STRETCH
Exercise Category: ARMS - HANDS, CLUB CONTROL - ACCURACY, FLEXIBILITY
Stand with right arms outstretched directly in front of chest palm down. Reach left hand out and hold on to right fingers. Gently pull right fingers and palm toward toward inside of wrist as if in a cupping position. The resistance that the left hand provides on the back of the right hand will create a stretch in the back of the wrist and forearm. Hold for 5 seconds then switch arms. Repeat each arm 3x. Do not force stretch!