Add a second stone and sacrifice both
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Add a second stone
Direct atari: not so good
Direct block: also not good
Black has just played the crosscut at . (Sakata calls vulgar style in the context of the discussed game, but that is another matter. There are plenty of occasions where this sequence is perfectly OK.)
It is clear that is going to be captured. Nevertheless Black adds another stone, in accordance with the proverb.
The thinking behind Black's line of play becomes clear in this diagram. Black has forcing moves at , and . Thanks to his larger sacrifice, he has a move on the outside () and two which seal off the corner. (Note that makes a a gote follow-up for White. See /continuation for further explanation.)
In summary: adding the extra stone increases the (marked) liberties of the black group. As White needs more moves to capture it, Black gets more forcing moves.
For the sake of discussion, if there is a stone, then White will almost always capture by (ladder), and almost never play at a. This leaves only the forcing move at b, but the block at c is not sente.