This bent four shape has a serious weakness: the cutting point at a and the marked stones without outside liberties. makes miai of the snapback at a and destroying the eye at b.
After ,
is atari, reducing Black's eye space in sente, and
kills. If the marked stones had one or more outside liberty, Black is alive.
is an example of a bad atari which should not be played.
connects, and now a and b are miai, so Black lives.
is met by
(or a) to secure two eyes.
(For advanced readers: is slightly better than a because Black has two ko threats here after two successive White moves, which are possible in a ko fight)