Six Kyu Noseki
But then, one time, I was playing against an opponent who played the 5-4 point, and instead of playing the press that I used to favor, or attaching at a, he attached on the outside, at b. I played what seemed intuitive for me, and it ended up with a very bad result. I checked Essential Joseki, and indeed, the play that was "intuitive" for me turns out not to be there, meaning that it's probably wrong (although Essential Joseki is, of course, far from being a complete listing of all joseki).
Since the wrong play seemed, at first glance, to be sensible to me, I reasoned, perhaps it would seem to be reasonable to my opponents. And indeed, my opponents play the wrong move more than 50% of the time at the 6k* level, allowing me to reap unreasonable advantage by playing the outside attachment.
Here's the mistake:
It looks good at first , because White is expecting this continuation, which gives White big territory in the corner, and Black outside influence in the wrong direction (since his choice of that 5-4 point should generally indicate that he was going for influence down the left side, not the top):
Really, though, Black plays a two-step hane, trapping White in the corner.
Even if White a, Black b, White c, Black d, White's corner is just 12 points, while Black's outside influence is considerable. has some aji, but if White plays the hane and connect a and c, Black could always play d at e instead to remove that aji, in return for allowing White the possibility of an endgame clamp at f. Or, he could use to capture , at the expense of his left side influence.
This is the usual response to the outside attachment:
If White lures out her cutting stones, Black starts a ladder at and White has to defend at . Normally, Black should not fear being cut this way, but if the surrounding area greatly favours White, he may also choose for the previous diagram.
May I point out that "it looks good at first" violates basic technique. Permitting a hane at the head should always look bad at first, then maybe be playable, like in the avalanche joseki. See also joseki as a source of bad habits. --Dieter
Checking my game collection, I found that this has actually been played a few times among professionals.
Out of over 20,000 games in the MasterGo database, the position occurs 182 times, of which is played 4 times (2%). However, in all those cases White had some support down the left side and cut with here to start a fight.
However, this position also occurs after another sequence:
namely here as a ladder breaker, against .
This gives two games where the hane is played: