They go under the knee cap and put pressure on the patellar tendon. Supposedly they keep the knee crap from tracking improperly. Here's a picture: I don't think all of them have the cup-like plastic thing. But all of them do put pressure on that tendon underneath the knee. Long story short, I had knee surgery to remove broken cartilage and to release the lateral renticula. My knee still doesn't track right and I'm getting pain after I work out. Do these things work?