How does MP eater work?

For any magic attack you do, you have a chance of draining a good amount of MP from the enemy. While it does nothing to the enemy, this basically means with max MP eater, you would only be able to drain so much MP from 1 foe. Every magic attack you do, there is a 30% chance that you would gain MP from the enemy. This includes magic claw, energy bolt, and even the cleric's "Heal" attack on undead monsters. MP Eater is especially effective with mob attacks such as Heal or Lightning, for obvious reasons.

Actually, it seems that if you drain all the mana from a magic casting monster, it can no longer cast its attack.
http://strategywiki.org/wiki/MapleStory/FAQs