mgoetze: threatens to make a second eye at a. However, white cannot prevent this - if white plays a, black b makes a second eye. (White trying to capture b would lead to a snapback.)
demetria: It looks to me as if black has to play at B. If he doesn't, then white takes it and black has only one eye.
mgoetze: How is this a failure? Black still has an outside liberty, and as soon as that is filled, black removes the white stones in the corner by playing 1-1.
demetria: if black can play 1-1, then why does he have to wait for white to take the last outside libery? Can't he just do it now? If white knows the stones are safe, why agree to kill them by taking that liberty?
mgoetze: Taking that liberty doesn't kill the black stones, because black can answer by taking out the white stones, which gives two eyes. There is absolutely no need to play that out right away.
Karl Knechtel: What mgoetze means here is that black can capture the white stones any time, and is not forced to until white fills an outside liberty. The key point is that black is safe because the black group has that liberty in addition to its one-point eye (which is all white has). Later (in endgame), filling the outside liberty is sente for white.
demetria: After , a or ''b' will save Black.
mgoetze: Which is exactly the reason why White won't play here, but instead at b. After that, black can't play at a to make a second eye because that would be putting his own stones in atari, white captures at 2. So black is dead.
demetria: Well, if white plays b, then black has only one eye and is dead (no snapback). If white plays at (of the first diagram) then black plays at a which looks like this:
Black secures both the eye and the snapback. Whith the placement of , black has miai for life, but if black plays at , then black's only chance is hoping that white won't play at .
Karl Knechtel: This is exactly the situation mgoetze describes - white at b, black at a. Notice that the black group is in atari now. A play at the point I have circled on your diagram captures the black stones.
In life and death, there is no "hoping", only reading. :) The "main line" is what results from both players making the strongest plays.
mgoetze: Oh man, this diagram is a great candidate for Spot the Atari. And here's the solution:
demetria: I didn't know that White at b was still a snapback: should study harder.