Keeping your torso rigid and fighting rotation, reach one hand up and tap the opposite shoulder (B), place the hand back down and immediately repeat with the opposite arm.
Assume a strong plank position, arms straight, hands stacked directly below your shoulders (A). Keeping your torso rigid and fighting rotation, reach one hand up and tap the opposite shoulder (B), place the hand back down and immediately repeat with the opposite arm. Repeat, alternating arms in quick succession.