Elemental Mastery : "invert elemental resitances" What does that mean?

The new Elemental Mastery inverts enemy resistances (1/4 chance) - But what does this actually mean?
Do we have any clarification/explanation made by GGG?


IMHO there are a few ways for this "inversion" to work:

a) 75 -> 57
18 -> 81
Looking at the Monster Ele-Res Table I am thinking that this is how they do it (certain numbers make more sense now).

b) 75 -> -75
This seems to be the "favored" way in community discussions.
Gameplaywise I'd expect to see bigger chunks being removed from the HP-bar of enemy bosses if this was the case.

c) 75 -> 1/75 -> appr. zero
0 -> 1/0 -> infinity (probable workaround -> zero stays
zero)
The math way - very improbable and not observed in gameplay
(and this way they could just have written "sets the enemy res to zero" 1/4 of the time :D XD )

Which one is it??!

And: ... when is the inversion applied?
After/before deducting exposure/curses/pen etc.?

b) is correct.

It is inverted before penetration, and probably after exposure but I'm not sure.

I trust PoB is accurate and it does tell me this mastery is nearly as powerful as using one (and only one) elemental hex. If you stack Ele Weakness with Frostbite then no doubt the two hexes win outright. Don't use this mastery if you plan to use even one hex. It just makes it worse.

But if B ist right and the resistence gets lowered by my curse to below 0 then it gets + resistence when the 1/4 chance triggers. IMO