keima slide and ogeima slide
Discussion originally on answer keima with kosumi.
This is another common diagonal response, to a keima slide.
But when slides from one line further, the jump B is the usual response.
I learned this years ago. Frankly, I have forgotten why. Yesterday I saw Prof. Teigo Nakamura 6-dan. He has the most encyclopedic go knowledge of any amateur I know. He had forgotten why, too. ;-) But he spent a few minutes playing with the position and came up with the answer.
If White protects with , Black has sente; but she threatens to jump in at a. Note that Black had responded at a instead of the kosumi (marked), he would still threaten to play at b, but White would not have a big threat after .
(Moved from answer keima with kosumi.)
Firstly, there is no 'shape-based' rule.
If one looks at this sort of pattern, just somewhere on the side, then Black a and Black b are both commonly seen.
If one specialises to the case of the small high enclosure, then the diagonal move answer is more popular:
is more popular than Black a.
These come from database search: they may contradict the book I read.
Looking, as one should, into the game context, the diagonal move answers do seem to be in the type of position where Black wants to take sente.
There may be something in the idea that the one-point jump answer here invites and , at which point Black would want to add another stone here.
In the keima slide case, White's immediate cut with and is possibly tactically. (White would like a good ladder for this, but perhaps that isn't a precondition?)
is given as correct by Kano. In three pro games, Black instead played tenuki from the corner.
here is given as correct. This is more plausible: a low-ranked Korean pro has played a, but as a rule this is what one sees.