Stretch out the spear — This was, either, for a sign to his host present with him, to stop their flight, and make head against the pursuers: or, for a signal to the liers in wait, or, as a token of God's presence and assistance with them, and of their victory.
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.