No doubt Joshua had ascended the heights, most likely those to the north of the valley, so as to separate himself from the flying Israelites on the lower ground, and to be visible to the men in ambush behind the city. He now, at the command of God, gives the appointed signal to the ambush.
Related Commentaries of Joshua 8:18
Joshua 8:18
18 And the LORD said unto Joshua, Stretch out the spear that is in thy hand toward Ai; for I will give it into thine hand. And Joshua stretched out the spear that he had in his hand toward the city.