keima slide and ogeima slide
Discussion originally on answer keima with kosumi.
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.)
Charles Matthews I have wondered about this, having seen something very similar in an old Japanese book on tesuji.[1]
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?)
[1] A 1955 book by Kano Yoshinori. Here are the actual positions.