HOW: First get into a normal sideplank position with your top foot in front of your bottom foot. Engage your adductors and push your top foot hard into the ground. Then take your bottom foot off the ground so just your top leg is holding your body up in the air.
FEEL: You should feel all the muscles in your adductors working hard to maintain your position, in addition to your shoulders and core.
COMPENSATION: Keep a straight line between your shoulder, hips, and knees.