forces White to play (else Black gets his second eye by playing at ). Then Black captures White's stones in oiotoshi with and lives. See also Connect-and-die pyramid four gives one eye in gote.
ProtoDeuteric: If white knows about the impending oiotoshi, she would probably connect out so that she loses fewer stones, as well as having more time to play elsewhere on the board that is more important. at either "a" points is a gote necessity.
Bill: After in this diagram White has no ko threat. In the first diagram she has one. Since the net score is the same, White should capture .
ProtoDeuteric: How would White capture after a at either "a" point?
Bill: I meant, instead of connecting.