Targets: your abs, obliques, butt, and arms.
Start on your hands and knees, with your shoulders stacked above your wrists, hips above your knees, and toes tucked under. Balancing on your left palm and right knee, lift your left knee out to the side so your thigh is parallel to the bed. Keeping your ankle level with your knee, swing your knee forward as you tap it with your right hand. Then, swing your leg backward until your left ankle passes over your right leg on the bed. Simultaneously, reach your right arm back to tap your left heel. Continue alternating between left knee- and left heel-touches for 30 seconds, then repeat on the opposite side for 30 seconds, this time swinging your right leg and touching with your left hand.