smart. Spoiler/summary: they use a "leap smear" to keep code logic from breaking.
Instead of making code encounter the same second twice or not encounter a certain second, they smear the extra second over several hours beforehand through the central time server; by the time the leap second comes you're already sufficiently ahead/behind. (My comment: This works because the granulatiy of the time isn't that low anyway, so obviously no code can rely on it. Therefore, if code is correct without the smear it will be correct with the smear.)
Instead of making code encounter the same second twice or not encounter a certain second, they smear the extra second over several hours beforehand through the central time server; by the time the leap second comes you're already sufficiently ahead/behind. (My comment: This works because the granulatiy of the time isn't that low anyway, so obviously no code can rely on it. Therefore, if code is correct without the smear it will be correct with the smear.)