Atari against the group of marked stones. Black has to reply at or a, and White connects at
. The eye becomes false and Black cannot live.
Jacob4Jesus: I think in the first diagram is a mistake. If
is played here then
can only be here and
secures three eyes.
symplicity: Count how many liberties each of black's groups have after
. Because
can do something different.
stoned: would be at a, removing the upper group of stones, so all of black dies.
ajisubarashii Black would reply at , capturing two white stones. Then the only way White can make progress is to play at
, as otherwise Black plays at
himself and makes two eyes. After White's
, Black captures with
. Then a and b are miai for Black to live.