Still studying "Invincible" I stumbled upon the following position in a two stone game against Shuwa:

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[go]$$Wcm39
$$ ---------------------------------------
$$ | . . . . . . . . . . . . 3 2 4 . . . . |
$$ | . . . . . . . . . . . 5 . 1 X O O . . |
$$ | . . . . . X . . . . X . . O X X O . . |
$$ | . . O , O . . . . , . . . . . X O . . |
$$ | . . . . . . . . . . . . . O . . X O . |
$$ | . . . . . . . . . . . . . . O X X . . |
$$ | . . . . . . . . . . . . . O O O X . . |
$$ | . . . . . . . . . . . . 6 X O X X . . |
$$ | . . . . . . . . . . . . . . X X O . . |
$$ | . . . , . . . . . , . . . . . , O . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . O . . |
$$ | . . O . . . . . . . . . . . . X . . . |
$$ | . . . . . . . . . . . . . . . . O . . |
$$ | . . . X . . . . . , . . . . X , . . . |
$$ | . . . . . . X . . . . . . . . X . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------[/go]

Now my question is, why does Black not just extend to the edge with ? If we continue just as in the game we get:

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[go]$$Bcm40
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . 2 1 . . . . |
$$ | . . . . . . . . . . . . . O X O O . . |
$$ | . . . . . X . . . . X . . O X X O . . |
$$ | . . O , O . . . . , . . . . . X O . . |
$$ | . . . . . . . . . . . . . O . . X O . |
$$ | . . . . . . . . . . . . . . O X X . . |
$$ | . . . . . . . . . . . . . O O O X . . |
$$ | . . . . . . . . . . . . 3 X O X X . . |
$$ | . . . . . . . . . . . . . . X X O . . |
$$ | . . . , . . . . . , . . . . . , O . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . O . . |
$$ | . . O . . . . . . . . . . . . X . . . |
$$ | . . . . . . . . . . . . . . . . O . . |
$$ | . . . X . . . . . , . . . . X , . . . |
$$ | . . . . . . X . . . . . . . . X . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------[/go]

This seems preferable for Black, since White's upper group has less eye shape and the threat for the corner is the same.

Black wants to prevent white connecting, and also wants sente.

Your suggested variation doesn't seem to be sente, as black doesn't actually have a followup to connect underneath - white could just answer a with b. Black does get b himself (or similar) to make the same threat whilst aiding his top group, but perhaps this is judged insufficient since white's atari lets him get away too easily and opens invasion points in that same top group.

Click Here To Show Diagram Code

[go]$$Bcm40
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . a 1 . . . . |
$$ | . . . . . . . . . . . b . O X O O . . |
$$ | . . . . . X . . . . X . . O X X O . . |
$$ | . . O , O . . . . , . . . . . X O . . |
$$ | . . . . . . . . . . . . . O . . X O . |
$$ | . . . . . . . . . . . . . . O X X . . |
$$ | . . . . . . . . . . . . . O O O X . . |
$$ | . . . . . . . . . . . . 2 X O X X . . |
$$ | . . . . . . . . . . . . . . X X O . . |
$$ | . . . , . . . . . , . . . . . , O . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . O . . |
$$ | . . O . . . . . . . . . . . . X . . . |
$$ | . . . . . . . . . . . . . . . . O . . |
$$ | . . . X . . . . . , . . . . X , . . . |
$$ | . . . . . . X . . . . . . . . X . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------[/go]

So my guess is that black considers getting the extension in the centre to be worth damaging his group on the top.

My thought is that the descent does not threaten to connect to L17 or to wrap around the white group from the bottom. It also seems to increase the aji at O16 by limiting white's liberties more, though black still couldn't exploit it right away.

Another important question here is does the descent to p19 alter the status of the white corner group? The answer is no, but if black has p19, how would you answer black s14 to make sure the answer is still 'no'?

With a hanging connection

Thank you all - those are very good reasons.

@ Uberdude: I would play 'a'. Shusaku later played 'b':

Click Here To Show Diagram Code

[go]$$c
$$ ----------
$$ X . . . . |
$$ X O O . . |
$$ X X O . . |
$$ . X O a b |
$$ . . X O . |
$$ . . X X . |[/go]

But that doesn't depend on whether hanes or extends, does it?

http://senseis.xmp.net/?L2GroupWithDescent explains why the L+2 group is not unconditionally alive with this descent. Now there is the interesting Tsumego to figure out why the J+1 group remains unaffected ; )

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Aah, crap! Yet another reason, why I'm not the next Shusaku. Thank you for the link, SoDesuNe.