is the vital point.
is to prevent seki.
After White captures the four black stones. It's a bent four, but...
White can't make two eyes with a due to shortage of liberties, and black is looking at the snap back at ''c'.
Making an eye around a in gote is not good enough; Black can play b and then d to make a ko.
emeraldemon: what about a play at e? This renders the snapback useless, doesn't it? Actually, as I look at it, it seems after black responds a, white must play b anyway. So maybe playing e gives one more ko threat to black for later?
White makes two eye by recapturing.
See also: under the stones
Since Black cannot live should he try to capture White, then Black's best play is to capture at , force , and live with . Should White prevent Black's two eyes @ , then @ makes an unwinnable capturing race for White (Black has too many local ko threats and is ahead in liberties.)
SuperDave: White doesn't need in the above diagram. White can make two eyes above if Black takes a move to threaten the eye in the corner.