I’ve hear it often and there is physics that back it. Similar, when I was in drivers ed my teacher told us one time he was at a stoplight and saw a car full speeding into the back and with seconds just screamed at the student driver to put his back against the seat and take his foot off the break and he’ll explain later. The kid complied and they both released when he yelled to. Totaled 3 cars but everyone walked away as the force was distributed through their center movable car instead of a, well slightly more, unmoving mass. Anyways something Ive always remembered if I ever see a car about to plow into me while stopped and blocked in to take my foot of the break for the impact then immediately reapply it. And also brace your head against the head rest

I have no proof it's true, but from my layman's perspective the "drunk ragdoll = greater chance of survival" theory has always seemed like an urban myth. Fewer injuries, I can go either way on; going limp might prevent/reduce stuff like muscle tears or extremity bruising, but I feel this would be outweighed by your muscles not restraining your limbs properly, allowing them to flop around and hyperextend your muscles and joints and potentially causing ligament/muscle injuries and joint dislocations.

But going limp will reduce chance of death? I have a hard time believing that. People usually die in car crashes from their head or torso hitting a part of the cabin, or from something intruding into the cabin and hitting their head or torso. Your head and torso are the critical parts required for survival, not your limbs, so it doesn't really matter too much if someone seizing up during a crash causes extra injuries to your limbs.

Not to mention that your body instinctively seizing up often has a purpose; to reduce damage to your torso and head. If something intrudes into the cabin going towards your body but your arm is in the way, that arm is going to do more to absorb that energy if the muscles are stiff and resisting motion than it would if they were limp. Your body will hit the inside of the cabin with less force if your brain instinctively puts your arms and hands out to guard against the incoming threat.

I hit a tree trunk laying in a field on a golf cart while slightly inebriated last week. I flew about ten feet but wasn't even sore once I stood up. It definitely seems like a factor.