is necessary, eventually. It can also be at a, or any other move that protects against the cut.
captures three stones.
Black recaptures at and
blocks. Next
connects at
.
Black has 25 points of territory, plus one prisoner.
White has 16 points of territory, plus 3 prisoners.
On the board Black wins by 7 and can thus afford a komi up to 6,5 points.
White takes the ko and has more threats, starting with the cut at a. So White connects the ko and Black wins by 6 points. Black has lost a point.
Playing here is the same as connecting at
in the initial diagram. White could play
, capturing the four black stones. If Black responds with
,
captures all seven stones.