Ah, now then you are confusing 2 issues here - IMHO - an ejection seat is a pre-meditated acceleration at a rate the body can withstand - I think it was 18g/sec but some of the later seats might be more. In this situation, i.e. the controlled burn over a set period, you need the straight back to allow the force to be transmitted equally down the spine - straight back.
In a helicopter vertical crash your probably going to get a much higher instantaneous g reading than this. This is because you have a much shorter time and small stopping distance. Thus, you absorb more energy per second or milli second. IN this situation you do not want it going straight up your spine because it simply wont cope - IMHO. Thats why you brace. Now, the brace position is different for different heli types but it tries to look after the kneck and the flailing limb syndrome. Tightish ball holding back of kneck and legs with other hand. However, word of caution, if you have crash seats you must alter the position of your legs to allow the seat to stroke down - otherwise it breaks your ankles if you tuck them under! Running away gets a tad tricky!
Remember though - if its in to the water think orientation of your escpae and combine it with your brace position so one hand goes straight to your exit, the other to your harness release and away you go as soon as everything stops thrashing. Unfortunately, this is all best taught as a practical exercise. Next airshow wander up to the loadmaster of a mil helo and ask, I'm sure they will demonstrate. Oh, and if you are in a big cabin type aircraft and are on a monkey harness and can't get to the seat then lie flat on the floor - spreads load.
However, these are my own opinions in case you try it and it doesn't work!