וְהַמּוֹעֵ֗ד הָיָ֛ה לְאִ֥ישׁ יִשְׂרָאֵ֖ל עִם־הָאֹרֵ֑ב הֶ֕רֶב לְהַעֲלוֹתָ֛ם מַשְׂאַ֥ת הֶעָשָׁ֖ן מִן־הָעִֽיר׃
Now there was an appointed sign between the men of Israel and the liers-in-wait, that they should make a great beacon of smoke rise up out of the city.—
Rashi on Judges
A time frame was pre-arranged At the outset, a signal was arranged between the Yisroelites and the ambushers, whereby the ambushers would raise continuous smoke from the city. The Yisroelites would thus understand that the ambushers had conquered Givoh.