The sequence of and is the key. Note that if Black captures the three white stones, the group will get a second eye and lives.
The rest is atari all the way.
Which ends in a shunt after . Ironically, the stone appears to be at a very beautiful position but does not help White at all.
If , Black lives simply by atari at and .
The direct atari at does not work.
See Kyu Exercise 5 / Attempts for more discussion.