Dochi Yose Tesuji
In a game played on November 24, 1705 between Honinbo Dochi (Black) and Yasui Senkaku (White), Dochi was losing by a few points in the endgame, when he suddenly played a brilliant yose (endgame) tesuji.
The invasion made Black 1 and 3 sente, because if White does not capture the marked stone with White 4, Black would play at a and make a seki.
The invasion does not work if it is played after the hane-connect sequence.
-- Eric Suh?
You may wonder whether White has no other way to play. Let us have a look:
The other reasonable move for White is to play 4 here instead of a. Black next plays 5 and 7. Is this sente, one may ask? If not, White has refuted Black's play.
Black 1 is an eye-stealing tesuji, as it makes the eye at a false, but White 2 is a good response and Black's attack runs into nothing. No matter how hard he tries, e.g. with Black 3 and so on, Black cannot get a seki, ko, or anything else.
The solution for Black is to play on White's vital point himself, with Black 1.
White 2 is forced, as a move here would be rather awkward for White. Next, Black 3 is tesuji and Black 7 establishes an eye in the corner. Thus the position becomes a seki.
As can be seen, if White did not reply, he would lose his corner territory (in a seki with Black). Therefore, White had to add another move to defend his territory after Black played hane-connect at the top border. The hane became Black's sente and enables him to win the game.
--Arno
Question: What would happen if black were to play 233 as follows:
After playing at 1, Black could respond to a with b, connecting by taking advantage of white's lack of liberties, or else play at a himself to make an eye in the corner... --sj
If White seals at b, then White has enough liberties to kill Black in the corner since Black will be short on liberties due to an abundance of false eyes. Played this way it comes out pointwise identical to way the actual game. --Patrick