Diagonal Attachment Follow-Up
If Black ignores White's marked move, White can make a tiger shape. This is considered very advantageous (when the Black stone is isolated as in this diagram). Therefore, ...
, preventing the tiger shape, turns the white move into a shape that isn't by itself ideal (see diagonal attachment - as inferior shape).
Often White plays hane like this, especially close to edge.
Under comparable circumstances, when eyeshape is at stake, one sees Black hane like this.
Alex: Eyeshape is only a bit of the issue. The big advantage of this over the other responses is that it gains local sente, since White is more or less obliged to play a. It is thus often played in positions like the one shown below.
Alex: This is from A Dictionary of Modern Fuseki, The Korean Style. Similar ideas crop up frequently. When Black approaches at , he accepts that White may be able to play again in the lower left corner. This is fine, as long as the result in the lower right makes it hard for White to gain advantage that way.
If White extends with , is the correct move. If Black slid immediately to the point of (the instinctive joseki for most amateurs), White could attach at a and get a good result.
is natural (but wrong in this position - the dictionary advises b), because answering at c would make Black overconcentrated. Black therefore exchanges for (to forestall W at d) and extends to .
The problem with this, for White, is that it is hard to decide on a good follow-up in the lower left corner; the usual move at e allows Black to live in the corner in return for strong influence, but here that influence is wasted on account of Black's very stable group on the lower side.
If the diagonal move is deliberately played, and not a weak player's diagonal it may have been with the purpose of making Black heavy. Black may then dodge White's intentions and jump lightly to .
Charles here is a recognised sabaki technique, when Black would otherwise become heavy here. Black accepts that White can cut now. See diagonal attachment knight's move angle play sabaki technique.