Why is this tsumego solution correct?

[Bill Spight]

There are different varieties of Capture Go. The only ones I know of have the winning condition that the first player to capture some given number of stones wins. Usually you only have to capture one stone to win, but Capture-2, etc., are also played. The more stones you have to capture the closer the game resembles regular go.

My own preference is for the one that starts on an empty board, with no passes, no suicide, and you can also win if your opponent has no legal play.

Anyway, Capture Go teaches players about counting liberties and also about seki. But with Capture-1 you can have a seki that might be dead under Capture-4, because you can kill if you can sacrifice three stones.