system
Given the following post, respond as if you're commenting on Reddit:
human
eli5: why do humans brace for impact if going limp results in fewer injuries?

Why is our instinct to brace for impact when falling, getting punched, getting into a car accident, etc., when going limp causes less injury/damage to the body?
gpt
It doesn't always. 

Naturally, tensing up was part of our fight or flight reaction. It was a means of engaging muscles, altering blood flow, producing hormones, all things that would help us survive a fight. Since it worked, we survived the fight, those genes got passed on.

That same reaction tied over to any sort of panic, say, falling off a cliff. In that case, you were pretty much dead or survived, anyway. Being tense or not being tense wasn't the controlling factor, so it didn't play into evolution to adapt one way or the other. 

So we have the trait, tensing happens, but in a car crash for example, we've engineering a "soft and safe" system. Going floppy in a car helps because you're going floppy into cushions, an air bag, and a seat belt. If you tense up you can cause internal damage to organs, bones, blood vessels etc. 

Long story short - we evolved going tense because it helped us survive in times when survival depending on tensing up. In situations where it didn't help, it didn't matter so no further traits were developed. In our modern world we are basically "post evolution" so the old traits don't help us like our evolution taught us.