HOW: Begin in a seated position with you back up against a wall. With the designated leg drive your heel into the floor until your heels get close your butt. Here I am using my socks on wood to slide my legs, key is to use a frictionless surface (ie magazine on carpet, pillow cloth on hardwood)
FEEL: The exercise should be working the muscles in the back of the leg, specifically the hamstring muscles.
COMPENSATION: Avoid moving your trunk as you bend and straighten the leg, motion should be isolated to the leg here.