Variation [1] discusses @
.
White 21 @ circled spot is atari. If Black a, White b catches him short of liberties. Variation [2] discusses White 11.
[1]
If Black 8 at here,
makes life with a potential bent four, which Black cannot spoil with an atari as when there were a stone at a.
[2]
If White 11 at here,
and
set up a snapback, denying White her second eye.