Come into a forearm plank position. Your shoulders should be over your elbows and your ankles over your toes, with your body in a straight line from feet to head. Place your feet about hip-width apart for stability and pull your abs toward your spine.
Shift your weight slightly onto your right elbow and extend your left arm all the way out to tap the floor in front of you. Keep your back flat and try not to rock your hips or torso to the side.
Put your left arm back down and come back to the starting position, again keeping the side-to-side rocking to a minimum.
Repeat on the other side, tapping the ground with your right hand.
This counts as one rep. Complete as many reps as you can with proper form for 40 seconds, followed by a 10-second rest.