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The following position is from a 1998 China-Japan friendship match, described in Shuko's book The Only Move by Hinoki Press.

$$Bcm31 Why B33?
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . O O O X . . . . . . . . . . . . . |
$$ | . . O X X X O . . O . X . 3 1 . . . . |
$$ | . O X , . X . . . , . . . a O X X . . |
$$ | . O X . . . . . . 4 . X . O X . O . . |
$$ | . . X . . . . . . . . . . b 2 . O . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . X . . . . . . . . . . . . . . . . |
$$ | . . . , . . . . . , . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . X . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . O . . . . . , . . . . . , O . . |
$$ | . . . . . O . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------

I cannot figure out any benefit to playing B33 as opposed to simply capturing at a. There is no bad aji after a around the top-right corner that I can find, and it seems to actually introduce bad aji for white around b. In fact, I would almost consider a sente, which would have prevented white from taking the big point at W34 (which he did take in the game).

There must have been some advantage to B33 that I'm missing -- what is it?

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Whoa, nice one. I got a slight idea, but I'm only 1d so probably wrong. –  mafutrct Nov 11 '11 at 19:06
1  
@mafutrct: Post your idea! Even if it's not completely right it will at least generate some good discussion :) –  Coltin Nov 11 '11 at 19:48
    
@Adrian Have you got a SGF of the game at hand? –  mafutrct Nov 11 '11 at 20:54
    
@mafutrct It's only given up to move 57 or so in the book, and GoBase doesn't seem to have it (based on a position search). Hopefully some kind soul with GoGoD and Kombilo can track it down based on the pattern in the top-right, I also wouldn't mind seeing the whole game. –  Adrian Petrescu Nov 11 '11 at 22:04
    
Are you sure you got the date right? I think I had a copy of The Only Move older than 1998. –  Laval Nov 12 '11 at 0:22
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3 Answers 3

up vote 8 down vote accepted

The extension is meant to attack the upper white group.

Looking at the whole board, the only solid territory black has is the upper right corner (the left side is open at the top and has an overlong extension at the bottom for territory-making purposes. The position is strong, which means it's good for fighting but not so good at making territory). The white, on the other hand, has a small but solid territory and influences on two sides. If black captures, white gets to attach at M18 and settle the weak group, in which case the strong black formation on the left side would be devalued by the lack of anything to attack. In that case, my intuition tells me, black would be behind (over-concentrated top-left, top left corner wall against a living group, the extension on the left closer to the stronger group versus the perfectly reasonable spread for white). The white group on the right would be slightly vulnerable, but not to peeping at 1 (see diagram below. Heavy ko with white capturing first).

$$Bcm1 Counter-attack
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . O O O X . . . . . . . . . . . . . |
$$ | . . O X X X O . . O . X . . X . . . . |
$$ | . O X , . X . . . , . . . X . X X . . |
$$ | . O X . . . . . . O . X . O X 4 O . . |
$$ | . . X . . . . . . . . . . . O 3 O . . |
$$ | . . . . . . . . . . . . . . . 1 . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . X . . . . . . . . . . . . 2 . . . |
$$ | . . . , . . . . . , . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . X . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . O . . . . . , . . . . . , O . . |
$$ | . . . . . O . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------

However, after black extension at 33, a white attachment at M18 can be ignored by black indefinitely. J16 in particular looks like a counter attack. If white J17, black H16; white is dead (I might be wrong on this):

$$Wcm1 Counter-attack
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . O O O X . . . . . 1 . . . . . . . |
$$ | . . O X X X O . 3 O . X . X X . . . . |
$$ | . O X , . X . 4 2 , . . . . O X X . . |
$$ | . O X . . . . . . . . X . O X . O . . |
$$ | . . X . . . . . . . . . . . O . O . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . X . . . . . . . . . . . . . . . . |
$$ | . . . , . . . . . , . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . X . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . O . . . . . , . . . . . , O . . |
$$ | . . . . . O . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------

Had black captured instead of extended, white gets the hane at N17, which significantly increases white's eyespace:

$$Wcm1 With capture, white has eyespace
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . O O O X . . . . . 1 . . . . . . . |
$$ | . . O X X X O . 3 O . X 5 . X . . . . |
$$ | . O X , . X . 4 2 , . . . X . X X . . |
$$ | . O X . . . . . . . . X . O X . O . . |
$$ | . . X . . . . . . . . . . . O . O . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . X . . . . . . . . . . . . . . . . |

In summary, aji is extremely important when you are attacking.

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1  
Nice explanation! –  mafutrct Nov 16 '11 at 16:44
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[Update: See end of post]

I'm only 1 dan, so I'm just guessing. My idea for that move would be to avoid the attachment at M18.

$$Bcm31 The board if black captures
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . O O O X . . . . . . . . . . . . . |
$$ | . . O X X X O . . O . X . . X . . . . |
$$ | . O X , . X . . . , . . . 1 . X X . . |
$$ | . O X . . . . . . . . X . O X . O . . |
$$ | . . X . . . . . . . . . . . O . O . . |
$$ | . . . . . . . . . . . . . . . . . . . |

B2 is the strongest response sometimes, but after W3 neither B 'a' nor B 'b' lead to a good result as white can aim at the atari at 'c'.

$$Wcm1 
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . O O O X . . . a 2 1 b . . . . . . |
$$ | . . O X X X O . . O 3 X . . X . . . . |
$$ | . O X , . X . . . , . c . X . X X . . |
$$ | . O X . . . . . . . . X . O X . O . . |
$$ | . . X . . . . . . . . . . . O . O . . |
$$ | . . . . . . . . . . . . . . . . . . . |

This means that black probably has to block white at B2. Now that marked black stone might be better at a instead. But honestly, I don't believe in this theory myself.

$$Wcm1 Locally, white might play either a or b next. Or jump to the center.
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . O O O X . . . . b 1 2 . . . . . . |
$$ | . . O X X X O . . O c X . a X . . . . |
$$ | . O X , . X . . . , . . . B . X X . . |
$$ | . O X . . . . . . . . X . O X . O . . |
$$ | . . X . . . . . . . . . . . O . O . . |
$$ | . . . . . . . . . . . . . . . . . . . |

Anyway, regarding the sente-ness of the capture of the single stone:

I think cutting at 1, aiming at the atari of 'a', is crude and helps white, as white can simply extend to W2 and fixed his shape, while black got virtually nothing.

$$Bcm1 Wrong direction
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . O O O X . . . . . . . . . . . . . |
$$ | . . O X X X O . . O . X . . X . . . . |
$$ | . O X , . X . . . , . . . X . X X . . |
$$ | . O X . . . . . . . . X . O X . O . . |
$$ | . . X . . . . . . . . . . 1 O a O . . |
$$ | . . . . . . . . . . . . . . 2 . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |

Instead, it may be more interesting to attack the whole group.

$$Bcm1 Ambitious. Maybe white does not connect at W2 in a game, though.
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . O O O X . . . . . . . . . . . . . |
$$ | . . O X X X O . . O . X . . X . . . . |
$$ | . O X , . X . . . , . . . X . X X . . |
$$ | . O X . . . . . . . . X . O X . O . . |
$$ | . . X . . . . . . . . . . . O 2 O . . |
$$ | . . . . . . . . . . . . . . . 1 . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . 3 . . . |
$$ | . . X . . . . . . . . . . . . . . . . |

Update: I asked several high dan amateurs. They universally agreed that they would have captured. The only reason for not capturing they could see was fractionally increased aji at M18.

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There is one simple reason that I think we have missed here. What if the capture becomes a Ko? If this is the case white can cause havoc in the top right corner and maybe even capture it. The connection is solid and prevents this from ever happening. O I forgot to add one thing. The two black stones are a floating group. If white attacks them then he/she can chase them around and this is bad for black. Especially if they are captured.

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I don't think this is an issue. To make it a ko, white whould have to play bad shape, making her group more heavy. And even if black simply defends, the exchange does not really help white. –  mafutrct Nov 12 '11 at 16:39
    
Regarding the counter attack (I think you means after the last diagram in my post), certainly, black has to be careful, but the fight is at least even so it's better than, well, not being able to start a fight at all. –  mafutrct Nov 12 '11 at 16:41
    
It takes more than one move to make it a ko. And one of the moves needed is useless. And you get only one move at a time in Go. So remember, scenarios that require multiple moves on your part is merely wishful thinking. Even moreso in this case, because the ko alone is of very little consequence for black, who can use the extra moves to slaughter the white group at the top and end the game. –  Laval Nov 12 '11 at 18:51
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