‘ And YHWH said to Joshua, “Stretch out the spear in your hand towards Ai, for I will give it into your hand.” And Joshua stretched out the spear that he had in his hand towards the city.'
This was clearly the signal for the ambush to attack. This would not be a signal to the ‘heel' or reinforcements (Joshua 8:13) but to the original ambush. The spear may have had something on it to indicate that it was Joshua's spear and he may have waved it preparatory to pointing it towards Ai. He had clearly taken up a place from which his signal could be seen. It had all been well worked out in advance. But Joshua awaited some indication from YHWH that the right time had come. Note that he then continued to hold out the spear until the battle was over (compare Exodus 17:11-12). This was the signal that the victory was YHWH's. It would give confidence to his men.