The brace position is only relevant if you don't have a shoulder belt, just a lap belt. The brace position is supposed to prevent/limit injury to your upper torso and head when they're thrown forward against something due to the sudden deceleration in a crash. What you essentially do is move your torso and head as far forward as they will go, either until your body can't go any further forward (the typical "business class" brace position where you're completely bent over, with your arms wrapped around your upper legs) or because you lean against the object in front of you (the typical "economy class" brace position where you lean against the seatback in front of you).
In an aircraft with shoulder belts, or four- or five-point harnasses your upper torso is restrained (assuming you actually wear the shoulder belt properly) and your head is too far from any solid object to hit.
- bolt upright, with their back and head hard against the seat back (stiff as possible)
- arms at their sides, braced as far as possible to help keep upright.
I'm not a safety expert but it seems to me that this advice may (note: "may") cause further injuries. In a significant crash, you will never be able to withstand the deceleration by muscle power alone. So:
- if you have pre-tensioned your muscles to keep your head hard against the seat/head rest, your head will be flung forward anyway. But then the tension in your muscles will then cause your head to be flung BACK against the headrest. If that headrest is not properly adjusted, or absent completely, this may actually cause a whiplash. (This is the main reason for having head "rests" in cars.)
- If you brace and lock your arms against the dash the forces are so great that you may actually break your arms.
My idea would be that you need to let the safety devices, particularly the seat belts, do their job. That means that they need to be as tight as possible. (Until it hurts and then some...) That's something which requires conscious effort if you do not have inertia-reel lap or shoulder belts, but even if you have inertia-reel belts you need to make sure that bulky clothing does not loosen them up. Assuming you're properly strapped in, a limp body will probably suffer less injury than a tensed up body.
And if you do have airbags, make sure there's nothing in the way that hampers their deployment.