Frozen Orb Damage.....specifics needed

[WarBlade]

Cold mastery "pierces" enemies resistance, or, in other words, negates the displayed resistance to cold.  Also, it does not, to the best of my nowledge, reduce a percentage of enemies resistances, for example:

We've already established what it does. <_<

CM gives 75% piercing
Enemy has 75% cold resist
In effect, the enemy has zero cold resistance against your attacks.

And that is most definitely not what it does.

In your example the enemy would have 18.75% Cold Resistance to your Cold Attack (if my calculation is correct).

It can also (and I've tested this) reduce enemies resistances to negative, effectively increasing the damage done to them.

You can't have tested very well methinks. <_< Cold Mastery will not push enemy Resistances into negative values.