Chris Hayashida: makes one eye in the upper left corner. At this point, a and b become miai for a second eye.
After reading the discussion below, it sounds more like b and c are miai for the second eye. c is the vital point for the first L+1 group.
Chris Hayashida: First, the easy case. If , then makes a second eye.
If Black plays instead, then it's more complicated. White ataris at , forcing . After that, White has the first L+1 group. White plays to make life.
If White tries to save the two stones, Black plays . White is left with a dead L group. It would have been better to sacrifice the two marked stones so that the whole group doesn't die.
Zook: I think this is an interesting variation too. At first sight, it looks as if a and b are miai to kill the outer eye and White is still left with a L-group in the corner. However, there are weaknesses in Black's position.
White now opens one of the variations in the L-group to make use of the weakness at the circled point.
captures two stones. If Black throws in, White captures his stones by pushing through at .
Zook: If at , I suddenly find a problem. Still thinking ...
Chris Hayashida: I updated this diagram to show Zook's problem. and falsify White's second eye. and can be swapped, and the eye is still false.
Chris Hayashida: What if Black plays at the critical point? White cannot immediate play at b. makes a and b become miai for life.