The Judan shares most of its structure with the other big titles in Japan - the winner of a sixteen member preliminary tournament plays a best of five match against the previous year's title holder. The Judan is unique though in using a double knockout format to determine the challenger. Anyone who loses a game in the preliminary knockout tournament goes to a parallel tournament, the losers' section. At the end the winner of the loser's section plays the winner of the winner's section to decide who becomes the challenger. The original idea for this is attributed to Fujisawa Hosai.
Commentary by Michael Redmond 9P
Published in the November 23, 2010 American Go E-Journal
In the previous Judan tournament I won against Hashimoto Yujiro 9p in the B final, and lost to Oya Koichi 9p in the A section. One win would have seeded me into A, but as I lost my first game in that section I fell back to B. This win puts me back in A. I think that section A will start in a month or so, and will be two rounds. A has almost 80 players participating, not counting Kansai Kiin which is separate at this stage. There are a total of 16 players in the Judan Honsen, and only 8 open seats, so after section A there is a final elimination section of about two rounds to choose 8 new players for the honsen. The final elimination includes 8 players dropped out of the last year's honsen, a few players from Kansai Kiin, and almost 20 winners from section A. They try to make the number of participants 32 so that the direct knockout system gives 8 winners in two rounds. In the honsen 8 players are seeded from the previous year's honsen, and there are 8 new participants from the elimination rounds.