Snapback
The stones are caught in snapback, a tactical situation beginners will learn to recognize early in their study.
Black can capture with
. (In practice, he would never do this except possibly as a ko threat.)
Threatening to capture stones in a snapback (or threatening to threaten to) can be powerful, as seen in this joseki. This diagram shows the result of a common joseki (for the 4473 enclosure) where White invaded at the 3-3 point under Black's 4-4 point. can hardly be omitted.
If Black tenukis, threatens to play at
, which would threaten a snapback, forcing Black to connect cravenly at a. Thus, Black has no choice but to play at
himself, creating a bamboo joint shape. But now White can move out with his cutting stones, starting a fight. (Of course, if Black is already strong on the top or left, the tenuki might be possible and has been seen in professional games.)
In some pro games, White has made the stand at before living. Black still has no choice but to play
, but now White will need to return to play
. Black may then surround at
, but the White stones retain some potential.
See also: