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is a double purpose move. It protects the cut at 'a', so now black is aiming at either 'x' or 'y' to kill one of white's groups.
The purpose of playing is that
offers some help when white tries to cut
and
off from the black group outside.
If at 'a', black 'b' is good enough (see diagram at the end of this section).
Because of , there's no eye at 'x'; the only chance for white to live is to kill some black stones.
Next, let's look at these white resistances one by one, from 'a' to 'e'.
Attempt to connect underneath fails, too. ( at 'y' works, but
here is a little better.)
Here White resists with . After 1, White must protect with 2, otherwise the group on the right dies if Black gets to play at 2. Then Black kills the other group with 3. White's resistance with 4 and 6 doesn't work.
After 7 in the previous diagram White can't capture the stones as the
stones have too many cutting points.
In some cases when white is strong around and black is trying to make two eyes, can be a good move that aims at either 'x' or 'y' next.
The position of the stone gave White an extra liberty, so the sacrifice of four White stones from
onwards is possible. As a result, Black only managed to capture four White stones.