British Go Journal No. 106.  Spring 1997. Page 43.
"Cho Kisei, you've entered byoyomi. Ten minutes left." That must be a familiar sound to Cho Chikun, who goes into byoyomi in most of his games. In the Kisei title match, the time limit is eight hours per player. There are no chess clocks; the time is monitored by a timekeeper. When a player has used all except ten minutes of his time, the timekeeper reads out the seconds, which is the literal meaning of byoyomi. The style is not the same as the byoyomi I remember at tournaments in Britain years ago. And I hear that these days, byoyomi has all but been replaced by Canadian Overtime. Still I thought readers might like to know how byoyomi is conducted in Japanese tournaments.
This article was prompted by the amazing amount of incorrect information posted on the rec.games.go newsgroup during 1996. My sources were videotapes of the Kisei and Meijin games of 1996 and a 1977 magazine article in Gekkan Gogaku on the topic of time keeping.
If the timekeeper utters "10", the player loses on time. If the player plays a move before then, no time is deducted and the next move is counted the same again. The final minute is reused every move. That is why game records (e.g., in the Igo Yearbook) often show that a player took 7 hr 59 m. It is not possible to take the full 8 hours of an eight-hour time limit. In the earlier rounds of the Kisei, the time limit is only 5 hours and many players take 4 hr 59 m.
The timekeeper does not say "Please play" as someone stated. The only time I have ever heard this was during a friendly exhibition game of rengo, between two teams of amateurs. One player in particular frequently failed to play within the time limit and had to be forcefully reminded. The organizers presumably did not want the game to end prematurely, as would have happened under professional rules. According to an article entitled "Time is the Enemy" in Go World #13, the timekeeper in TV games used to count "... 8, 9, play please". This practice was discontinued, however, because the seconds tended to stretch. Changing the count to "... 8, 9, 10" made things much more straightforward.
That explains why the current NHK timekeeper, Kobayashi Izumi, was replaced for the games her father (Koichi) played in. Izumi has gained a reputation for being an unflinching stickler for the rules. No slowing down or speaking more loudly from her, unlike some timekeepers.
Players who lose on time are generally very well-behaved. No McEnroe-type theatrics. I have seen at least two losses on time in TV games. Recently, Fujisawa Shuko was counted out in his NHK tournament game against Cho. He seemed bewildered and Cho looked rather embarrassed at winning by default. (Cho was just in the middle of making his corner live, which would have put him clearly ahead anyway.) Shuko said he thought he still had one minute left. He also said it was not the first time he had lost on time.
Another common misunderstanding about byoyomi is that the reusable minutes are special to the end of the game. This is not so. Throughout the whole game, no time is deducted if the player plays in less than one minute. The Japanese expression for this is literally "no time" (in katakana). So when a player is said to have played in no time, it actually means within fifty-nine seconds rather than instantly. The time is not kept by chess clocks, but by a human timekeeper who uses a watch. He/she keeps a detailed record on a special form, indicating the time taken, cumulative time used, and time of day for every move. These times only indicate whole minutes; the seconds are omitted. A move played in no time is indicated with a slash on the record sheet. When both players are down to their final minute, all the moves have slashes. This is known as "one-minute go". During big title matches, only one official is present for most of the game, acting as both game-recorder and timekeeper. Two lower-ranked professionals take turns. When one player approaches byoyomi, the second official comes in too, and one does byoyomi while the other keeps the game record. The referee, who is a high-ranking player, is usually only present to officiate the start, sealed move, restart, and end.
In one-day games having shorter time limits, for example five hours each, the period of byoyomi is only five minutes instead of ten. However, I believe the time is still counted in minutes (not 30 seconds as someone wrote). At least, it was in the twenty-year-old article I found in Gekkan Gogaku describing timekeeping for a minor game with a five-hour time limit.
During the 1996 Kisei, while Cho was in his final minute, he got up and left the room after playing his move. At the time, I wondered what would happen if Kobayashi Satoru played before Cho returned, but Kobayashi thought for a while and Cho came back. Was Kobayashi just being considerate? Was this an unfair imposition by Cho? In the very next game, while O Meien was giving the commentary on an exciting ko fight that seemed likely to decide the game, the camera switched to the playing room only to find both players absent, even though both were in their final minute of byoyomi. This was on nationwide live TV. It was an unusual enough event to prompt O to explain. The timekeeper doesn't start counting the time until the player is back at the board. That's provided he left in his opponent's time of course. Once his time has started, there's no interrupting it.