This is the previous lesson with the colors reversed: White's wall encloses a straight three, and the group is one move from life or death. The killer plays inside the space — a stone there keeps two liberties of its own, splits the eye space, and White can make only one eye however it answers. Strike from an end instead and White takes the vital point: two eyes, and suddenly your invading stones are the ones in trouble. One point decides the whole corner — the same point that made life a moment ago now delivers death.