I would imagine that there are certain situations, perhaps involving the martial arts, where bracing for impact is a good idea.
The rest of the time, not so much. If your car is about to hit a tree at thirty miles an hour, or the jet is about to slam into the wall of the Grand Canyon, it's not altogether clear that tensing all your muscles and preparing to be squashed is going to do you much good at all.
Worse than this, far worse, is that we brace for impact way more often than impact actually occurs. The boss calls us into her office and we brace for impact. The speech is supposed to happen next Friday and we spend a week bracing for impact. All the clenching and imagining and playacting and anxiety—our culture has fooled us into thinking that this is a good thing, that it's a form of preparation.
It's not. It's merely experiencing failure in advance, failure that rarely happens.
When you walk around braced for impact, you're dramatically decreasing your chances. Your chances to avoid the outcome you fear, your chances to make a difference, and your chances to breathe and connect.