You can find this exercise on page 23 of the book Synergistic Golf.
Exercise Type: STRENGTH
Exercise Category: ABDOMINALS, STRENGTH
Lie on the floor with knees bent and feet flat on the floor hip distance apart and 12 inches from hips. Lightly place fingertips behind ears for slight support of the neck and head. Inhale and prepare for the movement. As you exhale, pull your belly button to your spine and ribs rotate toward hips. Chin and head point toward thighs. Release down slowly and repeat 10 times slowly with focus on abdominals.