Ugh. Hock hobbles are a training device sometimes used in western pleasure that connect the horses hocks to their mouth so they get pulled with each step. It "encourages a low head."
(You know I don't go ugh about something unless I really have a problem with it).
I couldn't find a usable picture, so I'm going to link this page, which has a graphic of how they're used: http://sufficienthorse.blogspot.com/2016/03/rehabbing-broken-neck.html. They're sometimes confused with breeding hobbles, which also attach to the hocks, and are used to make absolutely sure the mare doesn't kick the stallion and injure him.