Judges 20:38

Now the appointed signal between the men of Israel and the men in the main ambush was that when they made a great cloud of smoke rise up out of the city

The Israelites had arranged with the ambush that they should send up a great cloud of smoke from the city,

Now there was an appointed sign between the men of Israel and the liers in wait, that they should make a great flame with smoke rise up out of the city.