You'll see double unders in just about every boxing movie montage. They're the move you see when the athlete's spinning the rope twice for every jump — and they're about as tough as they look. It takes a lot of coordination to nail, and it'll absolutely smoke your heart, calves, and forearms by the end. To do it, jump rope like normal, but jump an extra few inches higher on each bounce, and spin the rope twice as fast. Make sure you don't tuck your heels or your knees up — that'll just throw you off.