This is something I use when coaching (tramp) but it may be of help....
When a gymnast is bending their knees in a skill when they should be straight, a physical reminder to keep them straight can be helpful. I use tubular support bandage around the joint, which generally makes them more aware of the limb in the skill. I start with it doubled over, and then once they get them straight like that, take it to single, then take it off.