HOW: Start by lying on your back with both legs up on a wall or doorway. Move your hips as close to the wall as possible. Let one leg down as you keep the other leg straight up on the wall, relaxed. Move that leg up and down for the prescribed amount of reps.
FEEL: You should feel your hamstrings muscles stretching.
COMPENSATION: Keep the leg on the wall relaxed, don’t move the foot.