Come into a plank position with your arms and legs straight (in high plank).
Lower your left elbow to the floor where your hand was. Then lower your right elbow to the floor where your right hand was (now you're in elbow plank, on your forearms).
Come back into the starting plank position on your left hand and then on your right. This completes one rep. Engage your core the entire time, and try not to sway from side to side.
Continue for 30 seconds, then rest for 20 seconds.