The idea of join up is: if the horse doesn't do what I want then I'm going to make him work harder. It teaches a horse to listen because if they don’t they’re going to have to do more work than the initially would have if they’d just done what you’d asked in the first place. Some people say it builds a bond but really the horse learns that whatever you want them to do is easier than not doing it or doing what they want. For example, I want the horse to pay attention to me and stand by my shoulder but it won’t so I’m going to make it do work until it does, then when it does what I want I’ll reward it by letting it stop working. Doesn't scream natural to me.
It won’t work if the horse doesn’t know what you’re asking it. So if you want a horse to move a certain part of its body a certain way with a single cue you have to put the horse in that position, give them the cue, then reward them. Then once they know the cue, if they don’t send them out to do work. Join up is a crack pot idea that gives people a superiority complex and makes them think they are being integrated into the herd as a dominant member. It does teach a horse to listen to you but it does that by teaching them your way is easier.
The idea that it works to create a bond is crazy to me. If you’re going to use it be careful because you can teach a horse to resent you or try harder to prove they are the dominant one in the relationship. If it isn’t working then stop trying. Horses are bigger and stronger than their human counterparts and if you’re not paying attention to what’s working or not you’re going to get hurt.
Show me a horseman who hasn't fallen and I'll show you a man who has never truly ridden.