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.