Since the group is already sealed in, we first think about reducing the eyespace. If , then
reduces White to a door group which is dead.
White's obvious counter measure is to expand her eyespace by blocking at . Next,
cuts her stones apart.
maintains the cut and Black's stones will suffer from shortage of liberties on either side.
One possible continutation: Black has insufficient outside liberties to capture the cutting stones.
Borrowed from Dieter Verhofstadt/Why Fundamentals Matter, where you can find the failed attempts.