The clamp at kills.
Although onwards attempt to capture the White stone in a connect-and-die...
is a mistake, because retreats and makes an eye at a. Note that must not block at b, otherwise White plays at and reverts backs to the solution diagram!
To further demonstrate why Black lives, should attempts to crawl in, then is an atari on . This requires White to connect at , allowing to make his second eye and thus saving the Black group.
thanatos13: I think the point of this is to play in a vital point that connects. This looks a bit like the belly tesuji, that's what I look for first anyways.