[Kalzazz]

Interestingly, the 'spend 1 HP to avoid 1d+4' is one of the core mechanics of Earthdawn. In Earthdawn all active defenses (like GURPS, and are essentially skill based) require spending HP to allow you to roll them.

Your rolling against the attack roll to equal or better it.

And your armor absorbs damage, so if the enemy hasn't rolled so well as to get past your armor, the oncoming attack might well bounce.

And can be bad consequences for a sufficiently failed Active Defense.

So can be some intense thinking and turn players into quasi probability calculating machines as they ponder 'should I try to dodge or not'

So in GURPS terms

'The enemy made his attack by 3, I need to make my parry by 3 to have a shot. He seems to be a normalish human looking guy swinging a broadsword, and I don't think hes a weapon master or he would be throwing multiple shots or have a higher margin on his attack . . . . I am wearing light plate with fortify 1 . . . do I take the hit, or do I burn an HP to try and parry?'

Earthdawn also has a DX (and partially class level) based Passive Defense score. Which enemies need to roll to even try to hit you. Your Passive Defense assumes you are actually trying to defend, but active defense rolls are 'going above and beyond where your actually burning your HP and putting skills/powers in play'