HOW: Start on all fours with your knees under your hips and your elbows under your shoulders and hands on the ground. Put your toes into the ground, then push into it lifting your body slightly off of the ground. The wider you are the easier to balance, the more narrow the more challenging this will be. Keep your back flat and your core engaged. Bring one arm and the opposite leg up until your leg is straight out from your hip and your arm is straight out from your shoulder.
FEEL: With the leg kicking out you will feel the muscles on the back of the hip working, with the arm that is punching out you will feel the muscles on top of the shoulder and back of the shoulder blade working. You should feel your core muscles working the entire time.
COMPENSATION: Avoid rotating the trunk, arching the back as you raise your leg, or shrugging the shoulder as you raise the arm.