The eye at a is already secured. White needs to make another in the space around b. First, immediate capture is insufficient -
makes the eye false.
Second, capturing two stones by and
is also insufficient -
and
throw-in at
again makes the eye false.
So White chases Black longer with and then
! threatens to capture three to make an eye, forcing Black to capture with
. (Note:
at a is an unnecessary sacrifice, though it lives similarly.)
After , Black can't connect at
because of White's cut at
, capturing all six in connect-and-die. This trick works only near the corner - if the whole position is moved one or more lines to the right, White is dead.