system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Is it better to brace your body or stay loose in an accident/fall?

You ever go fly on a plane and the first thing they tell you in a crash is 'brace position', brace yourself against the seat etc to minimises injury. But then on the other hand you hear these miracle stories of cats surviving 20 foot falls because they stay loose and floppy when they land, or those guys who fall from huge halfpipes and can land that 'floppy' way to minimise damage. But in a car accident you are told to brace back against your seat? So which is better?

I can imagine bracing would be better for your brain. Minimise concussive damage and whiplash/neck trauama, but may increase the likelihood of broken bones?
gpt
Obviously every crash is different and some things will work better in one situation than they would in another, but the brace position in an aircraft crash, or bracing against a car seat have a very specific purpose.

In any collision there are multiple impacts.  In an aircraft crash there is the impact of the plane hitting the ground, you hitting the inside of the plane (seat, floor, controls, whatever the object is), and your internal organs (biggest concern being the brain) hitting the inside of your body.

The brace position is an attempt to remove, or at least lessen, the second of those impacts: you hitting the inside of the plane.  By placing your body against the seat in front of you, or against your legs (depending on the position used) you are already against the object you are likely to hit and will therefore push against it instead of impacting it.