Dieter: the following analysis can be found at 3-4 point high approach outside contact, tsukehiki hanging connection
White has the possibility of starting a ko by playing at a, however she should not embark on this ko lightly. If White loses the ko, her position will crumble. If Black feels a need to remove the threat of the ko, he exchanges Black b for White c.
Now the question:
If the ko is so troublesome for Black, then White should be in a position to increase its value by playing instead of a.
I assume that now Black can force White into a heavy position, as follows:
White can play this way too and get a good result, IMHO. So, the question remains why White would play lightly as suggested with c in the first diagram.
Dieter; That sure could be a reason. So if I understand correctly, White's move doesn't increase her gain from the ko as much as it increases Black's.
Charles Looking at the way the pros play it turns out to be quite interesting.
When is played, White tends to play at a if there is a white stone already in place at a circled point (or closer). If there is no such stone, or a more distant white extension stone (for example at one of the square points) White plays at b for good shape. Sometimes White just jumps out to p to defend the group.
dnerra: This has little to do with the ko, of course. Permitting the peep at b is a shape disaster if White doesn't have an extension yet. The group would become extremely heavy.
Dave: Keep in mind that the ko is not this straight forward.
If White tries to start the ko with , Black can answer at . White has to continue with if she really wants to play the ko. When Black cuts with he threatens to capture two stones instead of playing the ko. White has to play on the inside. However, does not work; is a well-known tesuji that captures the three stones. So...
Charles I think that if this were a tesuji problem, would be at a.
Dave Wouldn't it just transpose?
It seems as though White can squeeze Black from here with ideas like and IF Black refuses to play the ko.
So it seems like Black might as well start the ko at this point. However, now Black has a lot of internal ko threats, starting with a. This seems like a very difficult ko for White to fight.
Charles I think perhaps this isn't quite the right question.
Maybe the correct reasoning is more like this: here is a play that Black should take quite seriously. The temptation to answer at is high.
So this area is going to be something like 'sente for both', depending on the ko threat situation. If that seems favourable to White, then Black's play to forestall White is arguably more urgent.