Albert Barnes Commentary Joshua 8:18

"And Jehovah said unto Joshua, Stretch out the javelin that is in thy hand toward Ai; for I will give it into thy hand. And Joshua stretched out the javelin that was in his hand toward the city." — Joshua 8:18 (ASV)

No doubt Joshua had ascended the heights, most likely those to the north of the valley, so as to separate himself from the fleeing 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.