In this video Alex is demonstrating a Seated Hamstring Stretch. This is for a stretch of the muscles on the back of your thigh. This is a good stretch to perform both before and after surgery in order to help with knee straightening.
Sit in one chair, propping the heel of the leg to be stretched on another chair seat same height or slightly lower
Keep your back straight, head up
Don't allow the knee to bend up as you stretch
Lean forward from your hips/waist until you feel a gentle stretch at the back of your thigh/knee