HOW: Start with your hands on an elevated surface in a high plank position. Begin the exercise by letting your shoulder blades move in together towards one another while keeping your elbows straight. Return to starting position by pushing into the surface with your hands and spreading your shoulder blades apart and repeat. All while doing this - you're maintaining a good high plank position.
FEEL: This should feel like a shoulder blade workout along with challenging your arms and the rest of your body to maintain optimal high plank position.
COMPENSATION: Do not let your elbows bend, do not lose optimal high plank position, slowly control the movement with your shoulder blades.