Black's sacrifice at is the key to making an eye at the top edge in sente. Black's four in a row on the second line is good enough for an eye, and he can make the second with
in the space in the middle (a and b are miai).
If White responds to the cut with , she will be captured, solving Black's problems. This is a simple capturing race and Black is ahead three liberties to two.
The clamp at is wrong. White can connect at
, and to secure the eye on the top, Black must connect at
, this time in gote. After
, there is only one eye at the top. See six die but eight live for more information.