Two white stones sit on the third line with your stones pressed against their outer sides. Count and you find the flaw: each is down to two liberties, and one of those is the same point between them. A move on that shared point ataris both stones at once — the double atari (双打) — and White's reply can save only one of them. The typical error is atari-ing one stone from outside: it simply runs toward the centre, and the chance to take both is gone.