You can find this exercise on page 235 of the book Synergistic Golf.
Exercise Type: PILATES
Exercise Category: ABDOMINALS, FLEXIBILITY, LEGS - FEET, POSTURE, SHOULDERS, STRENGTH
Start in pushup plank position with feet and hands hip distance apart. Inhale to prepare. As you exhale, turn feet onto sides with toes pointing right. Lift up right arm in the air straight to sky. Balance is centered on left hand and between both feet. Pause. Inhale return to center. Repeat each side 8x with control. Focus on maintaining alignment through spine, hips and legs.