You can find this exercise on page 83 of the book Synergistic Golf.
Exercise Type: TOWEL
Exercise Category: FLEXIBILITY, LEGS - FEET
Sit on floor with right leg outstretched in front of body and left leg bent with bottom of foot resting on inside of right knee. Hold onto ends of towel with both hands and place it around bottom of right foot. Gently pull on ends of towel allowing toes to pull toward body stretching entire back of leg. Exaggerate the stretch by leaning chest forward toward knee. Hold 5-10 seconds at a time. Repeat as needed.