In the main line Black exchanges a for b first. If he doesn't, Black a will next be answered by White c, gaining her two points. Playing as in the diagram keeps White's points as low as possible without throwing away ko threats. (This is my fallible 1-kyu explanation, please review -- Dieter).
If Black persists in making White's second eye false, he suffers a double atari at 4.
If she takes the ko with the marked stone however, she loses her group if she loses the ko (Black 1 and 3).
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