Beginner Exercise 232 / Solution
hits the vital point. Now a and b are miai for the kill...
Black should not play either a or b now, since both are utterly useless. However, either move can be used as a ko threat later. If black plays one and white ignores it because the ko is more important, black plays the other and saves the otherwise dead group.
If Black makes an eye with , connects outside, leaving Black with only one eye.
If Black forms an eye with , White proceeds to falsify the other eye with and .
Playing atari with results in a ko, which is not the best result for White.