Matthew Poole and continuators on 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.

Or, thy banner: or there might be some Banner in the end of his Spear. This was prescribed and practised, either 1. for a sign to his Host present with him, to stop their flight, and make head against the pursuers: Or 2. for a signal to the liers in wait, as may seem from v. 19. who, though they were at some distance, might know this from persons whom they had set in some high and convenient places to observe Ioshua's motion, and to give notice from one to another, and that speedily, as is common in such cases, until it came to the whole ambush: Or 3. as a mystical token of Gods presence and assistance with them, and of their Victory; or as a mean by Gods appointment contributing to their good success, as the like posture of Moses lifting up his hands was, Exod. 17. 11, 12. which may be the reason why he continued this posture till the Enemies were all destroyed, v. 26. whereas if it had been a signal only, it was sufficient to do it for a little while. I know no reason why all these ends might not be joyned together.