What move didn't you expect, f18? It's another choice to blocking outside. Rather than the defending the corner normal would be to crawl with your pincer stone. The kick and descend can be a good exchange for white in that it makes the wedge weakness a bigger problem for black so he probably needs to come back to fix it, plus it makes invading the 3-3 harder. I suppose you play the kick from joseki memorisation rather than understanding why it is a good exchange, so this is a good learning opportunity!
Here is the textbook reason why kick can be a good exchange (and why black should maybe block the outside instead of descend, both are playable, and indeed checking the AlphaGo opening book it sometimes prefers descend to block). If you don't kick then maybe black will play for the moyo with 8 instead of the traditional joseki connection at
a (to which white would probably push at
b and black feels sad to lose the moyo and connection to the top right 4-4 stone). Iirc Cho Hunhyun liked to play this 8 in the 90s a lot vs Lee Changho (see
http://ps.waltheri.net/424406).
- Click Here To Show Diagram Code
[go]$$Wc
$$ +---------------------------------------+
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . X . O 1 5 . 7 . . . . . . . |
$$ | . . . O . . . 3 X 2 6 b . . . X . . . |
$$ | . . . . . X a 4 . . . . 8 . . . . . . |
$$ | . . . O . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
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$$ | . . O . . . . . . . . . . . . . . . . |
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$$ | . . . O . . . . . , . . . . . X . . . |
$$ | . . . . . X . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ +---------------------------------------+[/go]
If white then says "hey you didn't defend there, so I'll wedge" then this probably happens (another choice is
a to stop black playing there or attaching one left). The top left 3-3 is not white territory yet, black's 3-3 should live later, so black doesn't mind white connecting through with 13 so much as he gets sente and keep developing the moyo.
- Click Here To Show Diagram Code
[go]$$Wcm9
$$ +---------------------------------------+
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . X . O O O . O . a . . . . . |
$$ | . . . O . 5 3 O X X X . . . . X . . . |
$$ | . . . . . X 1 X . . . . X . . . . . . |
$$ | . . . O . . 2 4 . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . , . . . . . , . . . . . , . . . |
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$$ | . . O . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . O . . . . . , . . . . . X . . . |
$$ | . . . . . X . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ +---------------------------------------+[/go]
However, if black tries the same thing with white having made the kick-descend exchange:
- Click Here To Show Diagram Code
[go]$$Wc
$$ +---------------------------------------+
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . 2 . . . . . . . . . . . . . |
$$ | . . . . 1 X . O 3 7 . 9 . . . . . . . |
$$ | . . . O . . . 5 X 4 8 . . . . X . . . |
$$ | . . . . . X . 6 . . . . 0 . . . . . . |
$$ | . . . O . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . , . . . . . , . . . . . , . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . O . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . O . . . . . , . . . . . X . . . |
$$ | . . . . . X . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ +---------------------------------------+[/go]
Then now if white wedges the marked exchange is good for white as black can no longer live at 3-3, so white is making more profit here (white can just block
a at
b).
- Click Here To Show Diagram Code
[go]$$Wcm9
$$ +---------------------------------------+
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . B . . . . . . . . . . . . . |
$$ | . . a . W X . O O O . O . . . . . . . |
$$ | . . b O . 5 3 O X X X . . . . X . . . |
$$ | . . . . . X 1 X . . . . X . . . . . . |
$$ | . . . O . . 2 4 . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . , . . . . . , . . . . . , . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . O . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . O . . . . . , . . . . . X . . . |
$$ | . . . . . X . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ +---------------------------------------+[/go]
So probably black ought to defend, but then white can push out and is happy. A positive of defending is black can play
a, but white doesn't mind so much.
- Click Here To Show Diagram Code
[go]$$Bcm10
$$ +---------------------------------------+
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . X . . . . . . . . . . . . . |
$$ | . . . . O X . O O O . O . . . . . . . |
$$ | . . . O . . . O X X X 2 . . . X . . . |
$$ | . . . . . X 1 X . . . . . . . . . . . |
$$ | . . . O . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . a . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . , . . . . . , . . . . . , . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . O . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . O . . . . . , . . . . . X . . . |
$$ | . . . . . X . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ +---------------------------------------+[/go]