KarlKnechtel: threatens
at
which would make the second eye false; White must block. After
, White can't connect with
at
because of shortage of liberties; and with any other response
captures, preventing the requisite two eyes. Even with the capture and connection of
,
still destroys the eye, so White is dead.
The shortage of liberties threat which follows shortly after at a is the only key to solve this one. If Black omits the exchange of a for b White is alive after
.