The main line is the descent at
, which looks like it can live with a straight four. But with the correct sequence from
to
, Black is caught with a shortage of liberties, and is unable to play at a.
If
, then
occupies the vital point. Later, if Black plays at a, then White can play elsewhere.
The hane at
is interesting. If
is played wrongly, it can lead to a favorable approach ko. With the correct move, Black dies.