Hi all. Played a 9x9 game in which according to the post game auto AI analysis (free version) i had at some point a move in B1 (see diagram) that supposedly could keep my lead as black at +9.5. (Right after white’s capture move in E9, taking a black stone that was on F9).
Tried my best to see how B1 could work, but simply couldn’t. It seems like some kind of a throw in. Is it some kind of a ko threat play? Something else? Tested variations continuing from each empty intersection but nothing showed results. What is it?
There is a ko at f9. You need a ko threat to recapture. After b1, if white finishes the ko on top, you play a2 and will be able to capture the corner group. If white plays c1 to save the corner, you get to capture on f9.
B1 threatens to capture the corner stone with A2, then the whole group with C1. That is a pretty big threat, so white should capture the stone you just put with playing C1 themselves, but then you can capture the ko back with F9. I cannot see any big ko threats for white so the best follow up afterwards is proably just living in left upper corner, but you gained some territory and captured some stones as the result of ko fight.
The moves like B1 that you can use for opponent to spend a move so you can recapture ko are called as “ko threat“s.
ps- apparently my assesment is wrong and it is better for white to accept the loss of corner and take back the ko, either way white has to choose one or the other.
It’s not easy position because of kos. If you are not familiar with kos, maybe read about what it is first. (In sensai library for example).
Black want to take back E9 but it’s illegal (repetition is illegal) so he wants to force white to answer somewhere else. If so then he can take back E9 (no more repetition)
That’s why he should play at B1.
Now white has 2 choices
1- answer and eat B1. Then as said black can now retake E9. And the war of threats will continue (next white to find a move somewhere else which threaten more as what the ko is about, called a ko threat, so he can retake)
2 not answer B1 and instead finish the ko up (by capturing the 2 stones at D7 for example.
In that case white is safe up (ko is finished) BUT now black continues down by eating A1. That’s a new ko (check, white is in Atari, only solution is to retake (illegal yet)), and another ko bigger one for the life of the whole white group. Try to find a threat bigger for white is going to be hard.
Ko are quite a difficult part of the game play, no worry.
