HOW: Begin lying flat on your back with something under your heels to allow your feet to slide back and forth. Start the exercise by digging your heels into the ground and sliding your feet towards your butt. Bend your knees as far as you can, then straighten your knees to starting position, repeat.
FEEL: You should feel your hamstrings muscles working with this exercise.
COMPENSATION: Do not arch your back during the exercise.