ChessWhiz: is not joseki, as far as I can tell. How does White punish this move?
HolIgor: Let us see if Black did not punish himself already.
The purpose of is usually a sacrifice in order to get a corner with an exit to one of the sides. So, let us see what happens if White enters at san-san.
The question is now whether is well positioned.
In the usual joseki it is at a or b. In the latter case Black needs one more move to cap the white stone. does not seem to be placed very well as the eye making potential of the group would be, probably better with a.
Now does not stop White's propagation along the top, compared to a stone at a.
Alex: Yeah, but is part of Black's wall (albeit with a cutting point), whereas a would not be. I'd be happy with this diagram as B, so I'm not sure White's immediate san-san invasion is good. After all, if Black plays at b now, the result is not too different from if White invades san-san after a one-space high pincer, which we all know is not recommended, because the result is so much better for Black than with the low pincer.
It seems that a stone at a is placed more efficiently. Of course, if this is not the first corner, joseki interaction with other stones might change everyting.
Of course, White might not co-operate, and then I will have a hard time showing that was not an overplay...
I would start by claiming that I of course checked all ladders before playing (2 in the top diagram), and then see whether here might be a tesuji.
And the fight continues beyond the scope of both this diagram and this analysis :-)
Black could also play here. I don't see a good move for to fix the shape, however. Any shape guru to comment?
Still, under special circumstances, this could still be good for Black.
To come back to the original question: It looks very natural for me to start with and . How should Black continue?
To me, it looks like a more forceful move than the pincers a or b, and possibly making a better use of Black's wall in the bottom right.
White chose this sequence.
For the record, Black won by more than ten points (the center moyo remained almost untouched, solidifying in a single huge territory).
I'd conclude that this move is playable under specific circumstances (looking at a couple other games with this pattern, I suspect that the plan is to emphasize an existing strength/influence on the side where the second stone is played and in the center).
(For those with access to Jansteen's site (not everyone?) the link to the game)
This is a non-standard move in modern go but it is not a trick play. It is obviously outside oriented so if Black can get extra value from a wall facing along the right or toward the bottom, it can be a viable choice. It appears in older Chinese games dating back to at least the 12th century AD.
Pulling back at after White pushes through and cuts is one basic idea. However, it is too tame after challenging White with the fierce shoulder hit to start with.
The result to is judged better for White who has sente.
and here are a special strategy to keep sente in order to play down the left side.
However, Black is clearly thin and White is threatening to start something with White a, Black b, White c later.
If White switches to here, it stops Black from forcing White down as in the previous diagram; but it is a slower approach for White.
When Black pushes at , White is unsatisfied after because of Black's good move at . However, if White plays at herself instead of , Black will cut immediately at a and has interesting chances.
Black can try moving to here but after and White is set to make a lot of territory in the corner.
In the local situation White is better. However, as mentioned above, the original shoulder hit is usually based on the overall position so this may still work for Black.
This is more oriented specifically toward the top.
Again however, Black is going to have to cede the corner to White so locally this is going to hurt.
Locally the question seems to be what happens after Black plays this combination of and .
The ladders toward the lower right need to favor Black but if they do, this can lead to some heavy fighting that favors Black at least as much as White.
If is here instead, Black plays and waits for the opportunity to play around a, forcing White to respond in the corner.
ChessWhiz: Thank you, all, for your contributions.
Hunz: I can't find what's the best continuation for this:
xela: This looks complicated! I wonder what happens if white plays at either a or ''b'.
fractic: This should still work. Black probably has to give in at 6. And in the end is still not well placed and Black has played the bad exchange of -.