The Black group looks to be safe, but it has a serious weakness - the marked stones have no outside liberties.
! kills. After
, Black can't play at a for shortage of liberties. This trick is often seen (or threatened) in practice and called the "old mouse stealing oil" in China.
and
tries to form an extra eye, but
and
executes a snapback, which at the same time falsify Black's last hope at a.