I had a wide array of moves to choose from. The general idea is to reduce White's potential on the right side of the board and, at the same time, gain points where they are easier to get--the top or the top-right corner. To this effect, professionals preferred "a" or "b," usually playing "a" and then "b" in conjunction. Lee Chang-ho played "a" then "c."
- Click Here To Show Diagram Code
[go]$$Bc
$$ -----------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . f . . |
$$ | . . . O . . . . . , . . c b . O . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . X . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . a O . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . , . . . . . , . . . . . , . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . O . . . |
$$ | . . . . . . . . . . . . . d O . . . . |
$$ | . . . . . . . . . . . . . . X O e . . |
$$ | . . . , . . . . . , . . . . X , . . . |
$$ | . . X . . . . . X . . . . . . X . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ -----------------------------------------[/go]
It was also possible to tenuki, leaving the Black stone at the top-right alone for the time being. The best places to tenuki would be "d"--which expands the bottom moyo and fights for influence in the center--and "e" which would take a large corner and threaten to reduce White's moyo on the right from underneath the White stones. In addition to all the above options, Lee Sedol played "f" in one game, taking the corner right away. The exact situation on the board was this:
- Click Here To Show Diagram Code
[go]$$Bc
$$ -----------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . 1 . . |
$$ | . . . O . . . . . , . . . . . O . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . X . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . O . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . , . . . . . , . . . . . , . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . O . . . |
$$ | . . . . . . . . . . . . . . O . W . . |
$$ | . . . . . . . . . . . . . . X O B B . |
$$ | . . . X . . . . . , . . . . X , . . . |
$$ | . . . . . . . . . . . . . . . X . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ -----------------------------------------[/go]
Black already has the bottom-right corner which could be an important factor why Lee Sedol might have thought it was all right to take the top-right corner in exchange for additional White influence toward the right side.
As to the move in the game, the knight's jump is a relatively new move. Out of the 40 games in the database I could find, 30 of them happened in the last 10 years. Let's compare and contrast this move with similar moves:
- Click Here To Show Diagram Code
[go]$$Bc (A) The move in the present game.
$$ ---------------------
$$ . . . . . . . . . . |
$$ . . . . . . . . . . |
$$ . . . . . . . . . . |
$$ , . . . . . O . . . |
$$ . . . . . . . . . . |
$$ . . . . . . . X . . |
$$ . . . . . 1 . . . . |
$$ . . . . . . . O . . |[/go]
- Click Here To Show Diagram Code
[go]$$Bc (B) A move popular about 80 years ago, no longer considered joseki.
$$ ---------------------
$$ . . . . . . . . . . |
$$ . . . . . . . . . . |
$$ . . . . . . O . . . |
$$ , . . . . . , . . . |
$$ . . . . . . . X . . |
$$ . . . . . 1 . . . . |
$$ . . . . . . . O . . |
$$ . . . . . . . . . . |[/go]
- Click Here To Show Diagram Code
[go]$$Bc (C) A common joseki, with "a" or "b" next for White
$$ ---------------------
$$ . . . . . . . . . . |
$$ . . . . . . . . . . |
$$ . . . . a . . . . . |
$$ , . . . b . O . . . |
$$ . . . . . . . . . . |
$$ . . . . . 1 . X . . |
$$ . . . . . . . . . . |
$$ . . . . . . . O . . |[/go]
The problem with (B) is that the marked stone is on the 3rd line, leaving Black insufficient room to do anything substantial against the corner. As a result, either Black gets insufficient territory in the corner to offset White's gains (after White "a") or else Black's influence turns out to be insufficient to offset White's territory in the corner and on the top (after White "b"):
- Click Here To Show Diagram Code
[go]$$Bc (B)
$$ ---------------------
$$ . . . . . . . . . . |
$$ . . . . . . . . . . |
$$ . . . . . . W . . . |
$$ , . . . b . , . . . |
$$ . . . . . . a X . . |
$$ . . . . . X . . . . |
$$ . . . . . . . O . . |
$$ . . . . . . . . . . |[/go]
Therefore, (A) is better than (B) because White's corner stone is at 4-4 instead of 3-4.
As for the (A) vs. (C), the two moves are similar in many ways. In both cases, Black separates White's two stones, seeking to apply pressure against one or the other. As for the differences:
- Click Here To Show Diagram Code
[go]$$Bc (C)
$$ ---------------------
$$ . . . . . . . . . . |
$$ . . . . . . . . . . |
$$ . . . . . . . . . . |
$$ , . . . O . O . . . |
$$ . . . . . . . . . . |
$$ . . . . . X . X . . |
$$ . . . . . . . . . . |
$$ . . . . . . a W . . |
$$ . . . . . . B . . . |[/go]
Black's first two stones cannot be easily disconnected. These two stones apply greater pressure against the corner than toward
.
is not connected very well to the original two stones, however, enabling White to play "a" in various situations.
- Click Here To Show Diagram Code
[go]$$Bc (A)
$$ ---------------------
$$ . . . . . . . . . . |
$$ . . . . . . . . . . |
$$ . . . . O . . . . . |
$$ , . . . . . O . . . |
$$ . . . . . . . . . . |
$$ . . . . . . . X . . |
$$ . . . . . X . . . . |
$$ . . . . . . . W . . |
$$ . . . . . . X . . . |[/go]
By contrast, here all the three Black stones work well together and apply more pressure toward
. However, the original two stones are apply less force against the corner and aren't as well connected with one another.
I cannot really offer too much insight beyond this, however. I never played (A) before. Similarly, my knowledge about (C) is limited, given that I either invade the corner or approach the corner from the other side when faced with a close-low pincer. That said, as I was familiarizing myself with (A) and (C), I came to realize that these josekis are best when Black has a stone in the bottom-right corner (and absent White stones there). For example, Black cannot complain about the following result (So Yokoku vs. O Meien (b), 2004):
- Click Here To Show Diagram Code
[go]$$Wcm20
$$ -----------------
$$ . . . . . . . . |
$$ . . . . . . . . |
$$ . . O . . . . . |
$$ . . . . O . . . |
$$ . . . . . O . . |
$$ . . . . . X . . |
$$ . . . X . . . . |
$$ . . . . . O . . |
$$ . . . . X 7 . . |
$$ . . . . , 9 . . |
$$ . . . . 8 3 . . |
$$ . . . . 4 . . . |
$$ . . . 2 . 1 . . |
$$ . . . . . . 5 . |
$$ . . . . . . . . |
$$ . . . . X . 6 . |
$$ . . . X . . . . |
$$ . . . . . . . . |
$$ . . . . . . . . |
$$ ----------------[/go]
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EdLee
I'm not home, either, and won't be for a few more months! 
press could stay ahead of W:
:
or else he played his move by instinct. He had various moves to choose from, however. In any case, what he played is natural and good. I'll take a little bit of time to consider a few options I now have but I will likely play the following move, expecting the indicated sequence:
. Bear with me.
, where Black usually responded at 
:
could also be the following:
extend, here's one variation where B makes good shape with a series of sente moves:
:
and 
in sente:
is actually sente. This is important because my group will be much stronger after the following exchange:
can be at "a," as well.
at "b" or tenuki