‘ And the ambush arose quickly from their place, and they ran as soon as he had stretched out his hand, and entered into the city, and took it. And they speedily set the city on fire.'
The ambush had been there waiting hidden for over a day. They were no doubt relieved to see the signal and acted immediately. They charged the ‘city', overwhelmed it, and then quickly set part of it on fire to alert Joshua that they had succeeded.