Horse question--reining

Hi guys,

Quick question about reining horses/training. I was watching Clinton Anderson's video about training Titian and I noticed he had a black wrap around his one joint. Is this something specific to reiners when training horses? Why would you put a wrap on above the joint when training?

Video Here

A lot of reiners will put an upside down bell boot to help prevent knocking when asking for a spin. Usually I've seen it with polos/boots as well, but I'd assume this is the same ... just something to prevent them from knocking their knee