If there are no outside liberties, White can make a sente seki.
is the strongest attack.
is the vital point.
is necessary or White plays there (or to the right of it) to capture the corner.
If here,
makes a thousand year ko. This result is generally worse for Black than a seki.
at the 2-2 point invites another thousand year ko.
in this variation spoils the chance of a ko and invites a seki.
If there is at least one outside liberty, Black is alive with seven points.
Now and the throw in at
work.
creates shortage of liberties for White. (If there were no outside liberty,
would be self-atari.)
threatens to destroy seven points in sente (assuming capturing the corner would decide the game).
makes seven points in reverse sente. So,
is a typical ko-threat - but it is a bad one. In case of a ko the previous diagram supplies four ko-threats: that's one more than here.