Resistances , how do they really work ?

[lukep]

Damage affinity heals you for a portion of the incoming damage. It is calculated before resistances, but applied after you take damage. For example, with 20% darkness affinity, and 50% resistance, taking a 100 damage darkness hit, you would get hurt for 50 then heal 20.

[HousePet]

Damage Affinity heals you by x% of the damage you get hit with.
This does not reduce the damage in any way.
The healing happens after the damage is applied, so you could be killed even with healing pending.

[greycat]

It looks like damage affinity is pretty straightforward -- you get healed for x% of all damage taken, of that type. But only if you survive the attack in the first place.

...
Damn you, end-of-page!

[Infinitum]

Is there a specific reason why resist all, resist element x and damage penetration isn't additive/subtractive? Being able to quickly add up my resistance values would make gear swapping quicker, not to mention more intuitive.

[lukep]

Is there a specific reason why resist all, resist element x and damage penetration isn't additive/subtractive? Being able to quickly add up my resistance values would make gear swapping quicker, not to mention more intuitive.

Having resist all be additive is far too powerful. 45% resist fire + 25% resist all would give you 70% resistance (take 30% damage) instead of 59% resistance (take 41% damage), a ~35% reduction in damage if it was additive instead.

Resistance penetration reduces a percentage of resistance for the same reason, 50% penetration would be enough to completely ignore most enemies' resistance if it was just (resist - penetration).