None of the shorter rules apply, so every legal arrangement of this line is compared and the squares they agree on are settled.
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When to look for it
Only when none of the five shorter rules gives you anything. On the pictures here that never happens.
Why it holds
List every legal way the clue could sit in the line as it stands, then keep the squares that agree across all of them. It is always correct and always the last thing to reach for, because it tells you what without telling you why.
Try it
Mark every square this rule settles, then check.
Click a square once to fill it, twice to cross it off, three times to clear it.
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