Damage Eaters were introduced with SA. They give you mutant healing against damage of a specific type, but it doesn't just block the damage. So, say you have a damage eater that absorbs 5 fire. You get hit for 5 fire damage, the damage eater will heal those 5 pts back 1 pt at a time for the next few seconds. This assumes you didn't die.
The example isn't literal. I'll leave it to the math wizards out there to figure out how it really works and explain it to you... wouldn't want to steal their fun.