In a position like this:
- Click Here To Show Diagram Code
[go]$$Bc
$$ | . . . . . . .
$$ | . . . . . . .
$$ | . . . . . . .
$$ | . . . . . . .
$$ | . . . . . . .
$$ | . . X . . . .
$$ | . O X . . . .
$$ | . O X O . . .
$$ | . # O X X . .
$$ | . . O O X . .
$$ | . . O , X . .
$$ | . . . . . . .
$$ | . . . . . . .
$$ | . . . . . . .
$$ ------------------[/go]
The aji of the markes stone need not be expended in playing B8. It is a good move locally, but there is something better.
A little digression: remember this joseki?
- Click Here To Show Diagram Code
[go]$$Bc
$$ | . . . . . . . .
$$ | . . . . . . . .
$$ | . . . . . . . .
$$ | . . 7 . . . . .
$$ | . . . a . . . .
$$ | . . 1 , 2 6 . .
$$ | . . . 5 3 4 . .
$$ | . . . . . . . .
$$ | . . . . . . . .
$$ ------------------[/go]
...in which beginners are always encouraged to play 7 ( or 'a' ) as black because of this if they don't:
- Click Here To Show Diagram Code
[go]$$Bc 7 tenukis
$$ | . . . . . . . .
$$ | . . . . . . . .
$$ | . . . . . . . .
$$ | . . . . . . . .
$$ | . . 8 . . . . .
$$ | . . 1 , 2 6 . .
$$ | . . . 5 3 4 . .
$$ | . . . . . . . .
$$ | . . . . . . . .
$$ ------------------[/go]
Also, remember this?
- Click Here To Show Diagram Code
[go]$$Wc
$$ | . . . . . . . . . . . .
$$ | . . . . . . . . . . . .
$$ | . . . 4 . . . . . . . .
$$ | . . . . . . . . . . . .
$$ | . . . X . 3 . . . X . .
$$ | . . . . 2 1 . . . . . .
$$ | . . . . . . . . . . . .
$$ | . . . . . . . . . . . .
$$ ------------------[/go]
...in which 4 is played to avoid this:
- Click Here To Show Diagram Code
[go]$$Wc 4 tenukis
$$ | . . . . . . . . . . . .
$$ | . . . . . . . . . . . .
$$ | . . . . . . . . . . . .
$$ | . . . 5 . . . . . . . .
$$ | . . . X . 3 . . . X . .
$$ | . . . . 2 1 . . . . . .
$$ | . . . . . . . . . . . .
$$ | . . . . . . . . . . . .
$$ ------------------[/go]
I trust that those look familiar. They both have the same core shape, like this:
- Click Here To Show Diagram Code
[go]$$Bc
$$ . . . . . . . .
$$ . . . . . . . .
$$ . . . . . . . .
$$ . . X . O . . .
$$ . . . X O . . .
$$ . . . . . . . .
$$ . . . . . . . .
$$ . . . . . . . .[/go]
...in which the shape move for white is this:
- Click Here To Show Diagram Code
[go]$$Bc
$$ . . . . . . . .
$$ . . . . . . . .
$$ . . W . . . . .
$$ . . X . O . . .
$$ . . . X O . . .
$$ . . . . . . . .
$$ . . . . . . . .
$$ . . . . . . . .[/go]
Ok, that's the end of the digression. Now, back to the game at move 23:
In a position like this, you don't mind if he crawls:
- Click Here To Show Diagram Code
[go]$$Wc
$$ | . . . . . . .
$$ | . . . . . . .
$$ | . . . . . . .
$$ | . . 4 . . . .
$$ | . 3 2 . . . .
$$ | . 1 X . . . .
$$ | . O X . . . .
$$ | . O X O . . .
$$ | . X O X X . .
$$ | . . O O X . .
$$ | . . O , X . .
$$ | . . . . . . .
$$ | . . . . . . .
$$ | . . . . . . .
$$ ------------------[/go]
...because he is getting one point of territory per move, whereas every stone on your growing wall is worth more than one point.
Instead, remember that corners are bigger than sides, and think about the shape described above. Then look at the position again:
- Click Here To Show Diagram Code
[go]$$Bc
$$ | . . . . . . .
$$ | . . . . . . .
$$ | . . . . . . .
$$ | . . . . . . .
$$ | . . . . . . .
$$ | . . X . . . .
$$ | . O X . . . .
$$ | . O X O . . .
$$ | . X O X X . .
$$ | . . O W B . .
$$ | . . W , B . .
$$ | . . . . . . .
$$ | . . . . . . .
$$ | . . . . . . .
$$ ------------------[/go]
The shape move goes for corner territory:
- Click Here To Show Diagram Code
[go]$$Bc
$$ | . . . . . . .
$$ | . . . . . . .
$$ | . . . . . . .
$$ | . . . . . . .
$$ | . . . . . . .
$$ | . . X . . . .
$$ | . O X . . . .
$$ | . O X O . . .
$$ | . X O X X . .
$$ | . . O O X . .
$$ | . a O 5 X . .
$$ | . 2 1 . . . .
$$ | . 4 3 . . . .
$$ | . . . . . . .
$$ ------------------[/go]
And black reduces reduces white's possible corner territory - in sente.
( FWIW, depending on the rest of the board, 3 might be played at 4 or 5 or 'a', but that's beyond the scope of this post )