You can find this exercise on page 244 of the book Synergistic Golf.
Exercise Type: DYNAMIC STRETCH
Exercise Category: ABDOMINALS, BACK, FLEXIBILITY, LEGS - FEET, LOW BACK
Stand with feet together and arms close to ears straight overhead. While maintaining straight spine, kick right leg out in front while reaching down toward toe with left arm. Alternate sides 10x. It is important to keep erect posture and belly button pulled into spine.