You can find this exercise on page 65 of the book Synergistic Golf.
Exercise Type: TOWEL STRETCH
Exercise Category: BALANCE, FLEXIBILITY, LEGS - FEET
Stand tall on left foot while right foot is placed in middle of towel. Hold both ends of towel in right hand. Balance on left foot while extending right leg straight out parallel to floor in front of body then slowly pull leg out to right side keeping leg straight and upright posture. Hold for 10 seconds, then repeat with other leg. Opposite arm can help balance by extending out to the side or for more support, hold onto wall.