Endgame Tesuji4
This move can sometimes be seen in amateur games. It is almost always wrong. It can sometimes have shape problems when White plays at a. But even for pure territory, it is a loss:
Compare this with the result of correct play:
Once one has seen , it looks so obvious that one probably would not want to call it a tesuji anymore. But it is surprising when one encounters it for the first time. Since the sequence Black a through White d will almost certainly be Black's privilege, this is about 4 points better than the result of the monkey jump.
While the shape built by the marked stone may look akward, there is nothing White can do.
One tricky question is how early one wants to play this sequence.
Since the white move at is either sente, or creates the nerai of White a, Black's sequence is very big. Often, Black can play it as soon as he is sure that he does not want to enter at b instead.
--dnerra