The marked space is only good for one eye with White's stone placed as it is. White must prevent Black from making two eyes immediately with a play at , so the first move is obvious.
If Black tries to solidify an eye on the bottom, is double atari and will capture and destroy an eye, leaving black dead.
unkx80: Apart from HolIgor's comments below, think also how should defend. If you play
as in your diagram,
can play tenuki; that is,
actually dies in gote.
You mean the diagram beside this text I assume? It seems that if plays tenuki,
at
makes an eye out of the space at the top, and the four in a row on bottom should make a second eye, with White unable to connect the stranded stone.
unkx80: Yes, I mean the diagram beside this text. Hint: See also my comment below.
If Black tries to solidify an eye at the top, will cut, putting the marked stone in atari. Black cannot connect at a because of shortage of liberties, and will not be able to make an eye on the bottom.
HolIgor: If ?
Oops. How about this way? White must be careful not to fill in at move , since
at
would capture.
and
can be interchanged. After Black captures 2 stones...
unkx80: Actually can play tenuki.
... White recaptures and Black has no eye at the bottom.
Juha: After , couldn't White's next move be on any of the three marked points?
unkx80: Yes.