You can find this exercise on page 98 of the book Synergistic Golf.
Exercise Type: PRE-ROUND WARMUP
Exercise Category: FLEXIBILITY, HIPS, LEGS - FEET
Stand and kick right leg back bending at the knee and grasp ankle with right hand. Gently pull foot back while pushing hip forward to feel stretch in muscle between knee and hip joints in front of leg. Hold 10-15 seconds. Repeat with left leg. Club may be used as prop for balance.