Black must find a way to either play all of the points marked x or make an eye on the outside. is the only first move.
If white descends with , black cuts and the sequence to
is forced, allowing black to play one of the three points in sente.
is necessary for later.
Black continues by playing another of the three points with . Because of the marked stones, this is sente. If white plays
,
-
captures white's surrounding stones.
Herman Hiddema: This fails if white plays at
, black is short on liberties. Blacks exchange of
for
in the first diagram is premature.