WHY: The goal of the plank is to work on your core stability. This variation further challenges your core stability by forcing you to stabilize when you lift each hand off the ground.
HOW: Assume the tall plank position on your hands and knees. Starting with either your right or left hand, lower yourself to your elbow. Then do the same with your opposite side. Next, some back up to the tall plank position on your hands.
FEEL: You should feel all the muscles in your stomach working to stabilize you. You may also feel your shoulders, chest, and triceps working.