HOW: Follow the instructions in the video to properly tie the theraband around an anchor, and then to your ankle so that it doesn't slip. Once set up, try to straighten your knee against the resistance of the band. Think of lifing your heel up while pushing your knee down.
FEEL: You should feel your quadriceps muscles on the front of your thigh really working, especially as you move your foot further away from the starting position.
COMPENSATION: Do not use momentum of your body to push out against the band.