MDR - How does it work?

[lfd]

Correct me if I'm wrong, but I think it means that if you have a MDR of X, then every attack against you that does Y damage normally now does (X - Y) damage, where X - Y cannot be less than zero (i.e. it cannot heal you).  For attacks that come in waves, like firewall (I think), it decreases damage by X every second.  At least this is how I understand it , but I could be wrong.

That's correct. The distinction is between non-frame-based attacks that damage you once on impact (such as shaman's fireballs) and frame-based attacks that damage you continually whilst you stand around in them (such as blood lord's firewalls). Some attacks have both (such as the blood lord's meteors - initial impact damage AND frame-based fire damage in the afterflames). Generally if you stand around in frame-based attacks, you can see the critical hit indicator flash on your character quite rapidly. Some frame-based attacks, such as diablo's lightning, have penalties to MDR (making MDR less useful).