You can find this exercise on page 61 of the book Synergistic Golf.
Exercise Type: YOGA-STRETCH
Exercise Category: ABDOMINALS, FLEXIBILITY, LOW BACK
Kneel on all fours with hands directly under shoulders and knees directly under hips. Inhale and draw belly button to spine, pressing spine toward sky rounding back. Gluteals will be tucked under as well as head. Exhale and reverse movement by dropping belly button toward floor while head and hips lift up toward sky. Focus on keeping shoulders away from neck and ears. Repeat 10x.