From THTF P. 302: Chapter 9, "Meanings"

BRACE / TIGHTNESS: This comes from confusion about the feel that’s presented to the horse, but he can sure learn to operate without being braced or tight. If he’s unsure about the meaning of what a person does, he might get ready to do what he thinks he ought to do instead. This is where his self-preservation comes in, and survival is one thing those horses are sure about. The best thing to do is not to allow a brace to get started in the horse in the first place. But if the horse isn’t prepared ahead of time for what you’re going to need him to understand in the future, it’s liable to show up, all right. Probably somebody got in a hurry on the start and missed a spot, and that is likely what gets those things to show up in the horse. Those spots that get missed, why they aren’t the best, I’ll say that. But, if one’s been in there a while, you’ll take your time with him so it doesn’t get confusing for him when you start to get his thinking changed around. We try not to get in a hurry because it’s going to be confusing to the horse when we do. If his confusion isn’t taken care of, other things can happen that you don’t expect.