You can find this exercise on page 218 of the book Synergistic Golf.
Exercise Type: PRE-ROUND WARMUP
Exercise Category: FLEXIBILITY, HIPS, LEGS - FEET
Stand approximately 3 feet away from cart. Swing right foot up on cart so that right knee is higher than hip. Gently lean body forward placing more weight on right foot. Lift up on back toe to enhance stretch. Keep chest lifted pushing left hip forward to open hip flexors. Pause then return to start. Repeat 5x each leg.