The Israelites and the men hiding in ambush had arranged a signal. When the men hiding in ambush ¹  sent up a smoke signal from the city,

But when the signal, a pillar of smoke, began to rise up from the city, the Benjaminites turned around and saw the whole city going up in a cloud of smoke that rose high into the sky. ¹ 

the Israelites counterattacked. ¹  Benjamin had begun to strike down the Israelites; ²  they struck down ³  about thirty men. They said, “There’s no doubt about it! They are totally defeated as in the earlier battle.”