That's an interesting question, so I went out to the garage and checked my spine protector (I have a pro-tec). The thing provides a bit of resistance at an extreme bend, but only a contortionist would notice. I don't think you can expect any protection of that kind.
What it will do though, is spready any impact out over a larger area of your back, which I would think would mean that more muscles would be involved in absorbing the impact. Not going to keep you injury-free, but it should reduce the scope of any injury.
Honestly, I think a spine protector is as much a no-brainer as a helmet. I may occasionally skip wearing it, but not on park and jump days!
Wow, back-slam on the knuckle! I've knuckled any number of times, but only board-first. Hope it healed properly.