First, let me congratulate pak0 on his enterprising play. The crosscut has certainly made things interesting.

Second, one of my teachers, a former insei, told me that center play was fighting strength. This game illustrates that.

Third, FWIW, here is my top of the head thinking for Black's previous play.
$$Bc
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . O . . . . . . . . . |
$$ | . . . X . . . . . X O . . X . X . . . |
$$ | . . . . . . . . . . . . . . . . X . . |
$$ | . . . . . . . . . . . . . . . O O . . |
$$ | . . . . . . . . . . . . . X . . . . . |
$$ | . . . . . . . . . . 3 . . . X . . . . |
$$ | . . . . . . . 7 W W X . . . . . . . . |
$$ | . . . X . . . . 5 B O 2 . O . X . . . |
$$ | . . . . . . . . . 4 1 . O X X . . . . |
$$ | . . . . . . . . . . 6 . . . . . . . . |
$$ | . . . . . . . . . . . . . X . O . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . X . . . . . X . . . X . X . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------
- Click Here To Show Diagram Code
[go]$$Bc
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . O . . . . . . . . . |
$$ | . . . X . . . . . X O . . X . X . . . |
$$ | . . . . . . . . . . . . . . . . X . . |
$$ | . . . . . . . . . . . . . . . O O . . |
$$ | . . . . . . . . . . . . . X . . . . . |
$$ | . . . . . . . . . . 3 . . . X . . . . |
$$ | . . . . . . . 7 W W X . . . . . . . . |
$$ | . . . X . . . . 5 B O 2 . O . X . . . |
$$ | . . . . . . . . . 4 1 . O X X . . . . |
$$ | . . . . . . . . . . 6 . . . . . . . . |
$$ | . . . . . . . . . . . . . X . O . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . X . . . . . X . . . X . X . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------[/go]
First, the atari with

. This serves two purposes. It almost forces

, which makes the White stones towards the right heavy. Also, White may not respond with

later on. It is hard now for White to sacrifice those stones.

also provides some support for the

stone.
Next, the extension of

, which also serves two purposes, at least. First, it makes it even harder to sacrifice the four White stones, because doing so would give up more territory than otherwise. Second, it attacks the

stones. It may also offer some long range support to the K-16 stone.
Now, if

and

bolster the four White stones,

continues the attack on the

stones.
$$Bc
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . O . . . . . . . . . |
$$ | . . . X . . . . . X O . . X . X . . . |
$$ | . . . . . . . . . . . . . . . . X . . |
$$ | . . . . . . . . . . . . . . . O O . . |
$$ | . . . . . . . . . . . . . X . . . . . |
$$ | . . . . . . . . . . . . . . X . . . . |
$$ | . . . . . . . . O O X B . . . . . . . |
$$ | . . . X . . . . 4 X W 3 . O . X . . . |
$$ | . . . . . . . . . 2 1 . O X X . . . . |
$$ | . . . . . . . . . . O . . . . . . . . |
$$ | . . . . . . . . . . . . . X . O . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . X . . . . . X . . . X . X . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------
- Click Here To Show Diagram Code
[go]$$Bc
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . O . . . . . . . . . |
$$ | . . . X . . . . . X O . . X . X . . . |
$$ | . . . . . . . . . . . . . . . . X . . |
$$ | . . . . . . . . . . . . . . . O O . . |
$$ | . . . . . . . . . . . . . X . . . . . |
$$ | . . . . . . . . . . . . . . X . . . . |
$$ | . . . . . . . . O O X B . . . . . . . |
$$ | . . . X . . . . 4 X W 3 . O . X . . . |
$$ | . . . . . . . . . 2 1 . O X X . . . . |
$$ | . . . . . . . . . . O . . . . . . . . |
$$ | . . . . . . . . . . . . . X . O . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . X . . . . . X . . . X . X . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------[/go]
I suppose that if

, Kirby plans to sacrifice the

stone with

. Note that

, Black's previous play, makes this a relatively easy sacrifice to make. pak0's feeling that it was severe should be balanced by the realization that it makes the sacrifice easier than the other extension. Go is not easy.
