You can find this exercise on page 310 of the book Synergistic Golf.
Exercise Type: STRENGTH
Exercise Category: LOW BACK, POSTURE, STRENGTH
Lie face down on floor with arms and legs straight out. Inhale. As you exhale lift both arms and legs high as if flying. Head should be slightly lifted. Hold abdominals tight. Reach toes back and arms forward as far as possible. Hold for approximately 5-10 seconds. Then relax. Repeat 5x.