It's called "Nemesis Mode" and as per that catchy title it gives players a chance to control the game's monosyllabic, hulking brute.
From Capcom's official announcement:
Only available for the Xbox 360, Nemesis Mode sees the USS compete against the Spec Ops in this thrilling and brutal eight player battle to achieve the highest score but with the twist of being able to control a terrifying Nemesis. Track down the control unit and give the Nemesis orders to destroy the opposition.
So, sort of a "Capture The Flag" mode, only in this case there's only one flag, and he throws cars at people.
Still having trouble imagining this thing? Luckily Capcom also issued a trailer for the DLC gameplay mode which is now embedded at top-right. It's more "cinematic" than "in-depth look at gameplay," but still drives home the point that he controllable Nemesis is going to almost completely dictate the flow of the game. Both sides will start out scrambling for control of the big guy, but when one grabs that remote control, I have to imagine the other side swearing into their headsets and frantically backpedalling away from the new menace.
I'm still sort of wondering what Nemesis does prior to one side gaining control though. Does he wander aimlessly and punch people at random? Hide in a closet? Sit patiently next to the coffee machine and read the day's box scores, while sighing audibly?
As I mentioned in the headline, "Nemesis Mode" will only be available on the Xbox 360 iteration of Resident Evil: Operation Raccoon City. The game itself is scheduled for release on March 20, and Capcom promises that the DLC will be available for purchase from the Xbox Live Marketplace that very same day for 320 Microsoft points (that's $4 in real money).