It's really just for fun; few people pay attention to it. Basically, if you have the option selected in your OOC profile, you have a chance to be selected for the Deathmatch each time a new tournament starts. The server randomly choose people to participate, and the Deathmatch announcer starts each new round when he has time, to continue the tournament. Players vote each round, and the winner moves to the next round.
The overall tournament winner does not actually win anything, but their name is added to the winner listing. It's little more than a popularity contest. ^_^