You can find this exercise on page 207 of the book Synergistic Golf.
Exercise Type: YOGA-STRETCH
Exercise Category: BALANCE, FLEXIBILITY, HIPS, LEGS - FEET, LOW BACK
Start in lunge position with right foot out in front and left heel off the floor in back. Gravity is centered in middle and right heel. Hands are in prayer position in front of chest. Lean forward and rotate upper body so that left elbow is on outside of right knee. Maximize stretch by pushing left elbow against right knee, belly button pulled into spine, hold for 10 seconds, then switch sides.