You can find this exercise on page 212 of the book Synergistic Golf.
Exercise Type: YOGA-STRETCH
Exercise Category: BACK, FLEXIBILITY, HIPS, LOW BACK
Lie on your back with legs straight out in front of you. Raise one leg and bend at the knee to 90 degrees in knee and hip. Cross that leg over extended leg, while opposite shoulder stays on the ground. Feel a slight pull in the back and buttock. Hold and repeat on the other side. Repeat 2x.