You can find this exercise on page 141 of the book Synergistic Golf.
Exercise Type: TOWEL
Exercise Category: BALANCE, FLEXIBILITY, LEGS - FEET
Stand with ends of towel in hands behind body. Balance on left leg and lift right foot behind to rest in towel. Top of foot (shoelaces) should be in the middle of towel. Place both ends of the towel in right hand only. Slightly pull towel up toward sky while leg lifts back to administer more of a stretch. Hold and balance for about 10 seconds then switch legs. For more support, hold onto a wall.