What governs when tournaments start?

I was just wondering... Do they have a preset start date when they will start as long as there is at least the minumum number of players? Or some set period of time after the minumum number of players has been met? Or are they started manually by a site admin who notices a tournament's been full for a while?
As it is, I sign up for a tournament, and it seems like some random period of time later, it starts.

Originally posted by mikebind
Or are they started manually by a site admin who notices a tournament's been full for a while?

^ That's the one.

Right on Gatecrasher. Russ is on vacation!