HOW: Get set up on your stomach with one arm behind your low back and the other behind your head. Begin the exercise by squeezing your shoulder blades back and then lifting your hands off your back/head. Refer to the video for how the transition occurs - but switch positions with your arms/hands while keeping your hands lifted up in the air. Lower your hands down to your head/low back, repeat.
FEEL: This should feel like a shoulder and shoulder blade exercise working the muscles in and behind your shoulders and in between your shoulder blades. You also may feel a stretch in the front of your shoulder.
COMPENSATION: Try to limit motion to your shoulder and shoulder blade, fight the urge to arch in your low back or move from the rest of your body.