HOW: Get set up on your hands and knees with your hands beneath your shoulders, and knees under your hips. Push into the ground with your hands, lift your knees off the ground, and support your body while trying to keep your back parallel to the ground. While maintaining this position, alternate tapping your shoulders with your opposite-side hand. Repeat
FEEL: You'll feel your upper body, arms, core, and even your leg muscles working to maintain optimal position. You will especially feel your shoulder girdle on the side that you're supporting your upper body with your hand on the ground.
COMPENSATION: Do not let your body twist, try to maintain optimal form, avoid lifting your butt up high into the air.