The Grit of Resistance?

[Tommi]

And What I want to know:
How do each of these Effects work (the only 2 ways I can think of will be listed):
Amplify Damage (-100% physical resistance)
Decrepify (-50% physical resistance)
Lower Resist (-varies% elemental/magical resistance)
Conviction (-varies% elemental resistance)
Cold Mastery (-varies% cold resistance)

As far as I know, all these abilities other than Cold Mastery substract straightly from resistance. They also can cause your resistance to become negative, with a minimum cap at -100. Thus, it is just a big line of pluses and minuses like this: a+b+c-d-e, where different letters represent values of different abilities (resistances, skills, etc.). The final value is your resistance which is applied in the damage modification prodedure.

For cold mastery, I know that it somehow multiplies the total resistance by 1-Cold Mastery%/100, but I don't know exact details. I remember reading that it cannot lower resistance below 0% but I am not sure if the cap is before or after the difficulty penalty. Never played sorcs that extensively. ;)

Cheers,

Tommi