In this video, Alex is demonstrating how to perform a Heel Slide.
Slide your heel toward your buttock until you feel a gentle stretch in your knee and/or have moved through your full range of motion
Slide your heel back out until your leg is fully straight
You may want to skip to "Heel Slide Using a Leg Strap" video in order to assist yourself in getting an additional stretch at the end of your knee range of motion.