Not drain. He just wipes another player's emnity. I kind of want the best practice of "when to DPS as tank" to be backwards for the DRK. So he does his most effective DPS while he's tanking the boss. You spend your time as the Off tank building emnity, then when you tank swap, you hit that huge CD and damage buff right as you wipe the MT's hate and then with this huge lead of emnity just pop your Damage CDs and Bloodbath and go to town on the boss, while you maintain the boss' position by sniping the emnity of the other players in the raid.

It'd be a totally different flavor of tanking.