Sometimes when we struggle with something - with our horses - we just can't stop fretting/worrying/obsessing about it. We tell everyone, "my horse does x!" And, every time we ride, we think about x, worry about x, and expect our horse to do x. That's a pretty good description, I think, of a mental brace. And one of the things about it - it's a bit of a self-fulfilling prophecy. If you're expecting x, x is pretty darn likely to happen, even, or especially, if you're closely connected to your horse with mental feel. And then it becomes a habit, for both you and your horse . . . And we all know habits are hard to alter.
Red and I have struggled, for longer than I'd like to tell (years, actually) with our walk/trot transition. And not all walk/trot transitions - the very first one in a work session, and that one only. We're walking, I mentally ask for trot, his head pops up and body gets stiff (no trot), I go to a secondary cue, he fusses, sometimes contorting his neck and body, and trots. Sometimes, if he's feeling fresh, he leaps into canter rather than trotting - almost like he's bursting through a barrier. It happens almost every ride. After that first, ugly transition, the problem just evaporates - every other walk/trot transition is as perfect as can be, even if we stop working for a while, stand in the middle of the ring and pick things back up again. This makes it particularly difficult to work on, since it only happens once per ride. I can refine and polish my other walk/trot transitions as much as I want - they're pretty darn perfect - and nothing improves about that pesky first walk/trot transition. It's unlikely to be a soundness/soreness issue as he immediately has no problem whatsoever, even seconds later, with another walk/trot transition - those happen just with feel and I never have to resort to secondary aids.
I'm anticipating the balk/brace, and he's delivering it. Now we get to the "don't think about pink elephants" part . . .