In essence, it is a single elimination tournament where pairs of DMs have 24 hours to combine several surprise "ingredients" into a full scenario. The judge(s) then pick apart the two scenarios and pronounce a winner, who moves on to the next round.
The usual starting number is 8, the ending number is 1.
Prize: You get to put IRON DM in your .sig, lording it over those lesser masters of quickly created scenarios.