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.
For Black to connect at leads to a hideous loss, because of a shortage of liberties.
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.