Herman: Black has an obvious gote eye at a, and since there is no way to connect, black will have to find a way to make an eye in sente, the most obvious area being the lower left corner. I think the tesuji of
will do the trick
Although white can live with
through
, black can then cut at a, connecting his group and saving it that way.
unkx80: Isn't it better to play
at
, killing all the white stones?
Herman: You're absolutely right! I thought it led to ko, but it doesn't :)
If white connects with
as in this diagram, then with
and
, black makes an eye, and
and
become miai for the second eye.
This is better for white than the previous diagrams.
unkx80: I expect that the best white move to be
at
, black plays the same
, then
can be played at a.
Herman: Yes, that is better indeed
Bill: How's this?
unkx80: True also, I missed this.
A second question: in typical cases, isn't
better directly played at a?
Bill: Yes, you're right. :)
captures
, then White can force up to
. Later,
will normally be sente. Black gets 3 points locally and White gets 1 point at
.
After
White can force up to
. Later
is sente. The net local score is the same, since Black gets 2 points while White does not get 1 point at
. However, Black no longer has the sente in the center. As against that,
may not become territory.
BTW, the quick way to see that
in the first diagram is at least as good
in the second one is the fact that in the second one White can play
at 3, forcing
at 2 and transposing to the first diagram. :)