HOW: Start on your hands and your feet, have your hands positioned under your shoulders. To begin the exercise, assume a high plank position, while keeping your body straight lower your chest down towards the ground as far as you can. Simultaneously drive one knee up towards your elbow like a spiderman crawl, then push up and return to the starting position and repeat on the opposite side.
FEEL: This should feel like an upper body workout, your shoulders, chest, and triceps. You should also feel your core and your glutes/quads working to maintain a straight body from your heels to the back of your head. The arm opposite of the leg moving will be working harder as you have to shift your weight.
COMPENSATION: Keep your body in a straight line, don't hike your hips high in the air or let your butt sag to the ground.