Charles Ellicott Commentary Judges 20:38

"Now the appointed sign between the men of Israel and the liers-in-wait was, that they should make a great cloud of smoke rise up out of the city." — Judges 20:38 (ASV)

The signal that had been agreed upon.

That they should make. —Literally, multiply to cause to ascend. The actual words of the agreed-upon signal are quoted. For the word hereb (which is an imperative) some manuscripts read chereb, “a sword,” and this is adopted by the Septuagint (Cod. A). But the flash of a sword would not be seen at such a distance, and the word does not make good sense. Otherwise it would remind us of the shield, which was seen to flash in the sun as a traitorous signal from Athens to the Persians, just before the battle of Marathon.

A great flame with smoke. —The margin gives elevation for “flame.” It means a column of smoke, or “beacon.” (Compare to Jeremiah 6:1: Set up a sign of fire in Beth-haccerem.)