You can find this exercise on page 199 of the book Synergistic Golf.
Exercise Type: PRE-ROUND WARMUP
Exercise Category: FLEXIBILITY, LEGS - FEET
Swing leg up onto cart of platform that is high enough to challenge you, but low enough that you are able to keep base leg straight and tall. Lean over from hips focusing on good posture. Reach toward lifted foot and feel stretch from ankle to buttocks. Do not bounce! Hold for approx. 10 seconds then switch legs.