HOW: Start by getting on the ground with your hands directly under your shoulders and your knees under your hips about shoulder width apart. Use your core to stabilize your body and lift the other arm up straight up over your head with your thumb facing up. Repeat on the other side.
FEEL: You should feel the muscles in your shoulder working. You should also feel your core working.
COMPENSATION: Keep your back flat, don’t twist or arch it when you raise an arm up overhead. The goal is not to move when lifting your arm.