Sorry i cant find anything on the Arreat Summit about - enemy res anywhere. :\
Look Here: for most everything explained.
http://www.battle.net/diablo2exp/items/magic.shtml
Um now that i think about it it probably works just like cold mastery does:
Once mastery over this powerful elemental force is attained, a Sorceress achieves perfect command over this school of magic. Her freezing spells are now of such potency that those adversaries formerly impervious to their effects soon find themselves the surprised victims of a vicious, chilly assault.
Passive Effect - Pierces the cold resistance of your enemies.
Player vs Player - This will cut other Player's Cold Resistances, even when maxed.
Cold Mastery will only lower the resistance of monsters that are hit by the Sorceress, this bonus cannot be shared with party members.
Cold Mastery will not work against monsters that are Immune to Cold. So even if you have Enemy Resistance -105%, they will still be Cold Immune and you will still be unable to damage them. So what's the point of 100%+ you might ask? Well if they are NOT Cold Immune, it will lower their Cold Resistance down below 0% which essentially means they will take more damage. It's just like when *you* have negative resistances in Nightmare/Hell difficulty.
If a target has 75% Cold Resistance and you use Skill Level 1 Cold Mastery, the target's resistance will be 75% - 20% = 55%.