Every block in this line is already placed, so what is left must be empty — or the other way round.
2before
2after
When to look for it
Whenever you have just filled or crossed anything. It is the cheapest thing to check and it fires more than any other rule.
Why it holds
A clue lists every run in the line. Once those runs are all on the board, no square is left over — so the rest of the line is empty. The mirror case is just as useful: if the squares still open are exactly as many as the clue still needs, they are all filled.
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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Share of steps across 175 pictures: 55%