Start in a plank position, with your hands directly under your shoulders and your body in a straight line, abs engaged.
Keeping your core engaged, bring your right knee toward your chest, with your toes just off the ground. Return to a plank, then switch legs, bringing your left knee forward.
Keep switching legs and begin to pick up the pace until it feels a little like running in place in a plank position.
Continue that movement for 40 seconds, 50 seconds, or one minute, depending on the round you're on.