After White fills the dame (marked):
Black has to play at 1 to prevent seki. Black can only avoid seki by ignoring a ko threat, which costs too much to be acceptable.
Black can't afford to play the ko at a, and has to settle for a seki at b.
ProtoDeuteric: Why can't black simply make two eyes by playing here?
unkx80: Either way works.
Referenced byLateEndgameExerci...