1. Black's connection
First of all, Black has to decide whether to connect or not. For details, please read endgame connect or not.
If he decides to connect, there are basically three ways for Black to do so.
2. White's shape
When White has two stones on the second line, covered by black stones on the third as here, the hane tsugi is not possible because of Black's atari at . This situation is considered to be Black's endgame sente.
With three stones, there are some problems. White has to be careful of a throw-in at
. Therefore
at
is often correct (a case of One-Two-Three).
3. Black's shape
Black's shape can have a comparable weakness. Instead of connecting at ,
plays atari and
captures three or more black stones . Black can start a ko, but the ko entirely favours White (hanami ko).
Black should stretch once, then block.
With present, Black has another weakness to be careful of. If he blocks at
,
is severe. If Black captures at a, White b will at least capture two stones and destroy black's defence line. If Black b, White c is more than White could expect to begin with.
Here too, Black should cede a little territory. The empty triangle of is best when there is no eye shape at stake:
If Black plays in fear of bad shape, he loses two more points compared to the previous diagram.
If Black has present, no such weakness exists.
4. Avoid Ko
Q: Have you met a situation which it is possible to defend with an atari, but we should not atari? Why?
A: It is because of ko.
Drop back to avoid ko
If the ko is serious to you, or you know you are a ko loser, don't atari! Although this costs you 2 points, do you feel it is much better if you let W invade your territory (when you lose the ko)? :-)
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