Squeeze tesuji in joseki
This page serves as a comprehensive discussion of the squeeze in a reference position (joseki)
Reading question
Consider this position and focus on the cut at a. Your first reaction may be that Black doesn't need to protect the cut if the ladder works for him, and that he needs to protect if the ladder works for White. However this analysis is incorrect.
Black has a clever defense if White cuts. He starts with an atari at and then plays a net at
. If White now plays at a or b, she will be caught in an inescapable atari, so there is only one possibility left.
She plays atari at , and again at
when Black connects. Her plan is to run out at b if Black pulls out the stone at a to enlarge the upper group while keeping the atari on
or takes the stone at c.
However Black now plays a tesuji at , which sets up the squeeze. Since White is in atari, she has no choice but to capture at
.
Reference position
For reference, the starting position is the result of this rather common san-san invasion under the black hoshi stone.
This completes the position. Verify for yourself that against the cut neither the net above nor the one to come works if White has a stone on one of the marked spots. In go terms this means that the position has some bad aji for Black. He needs to remember this in the continuation of the game, because in some other sequence later in the game White may play on one of these spots for another reason and in doing so enable the cut.
The squeeze sequence above is also prevented by White taking Black's liberty at b. However, the following capture does not rely on this liberty.
Alternative
In position "White cuts!" there is another way to capture the cutting stone, which doesn't employ squeeze though.
One may verify that cannot escape. The strongest reply...
(, of course, threatens snap-back at a.
at a wouldn't be as good since Black would depend on more context, for instance, having a liberty at b.)
is admittedly a thank you move.