John Gill 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)

And the Lord said unto Joshua, stretch out the spear that [is]
in your hand towards Ai
On which was a flag, as Abarbinel and Ben Melech think; and which is not improbable, and served for a signal for the ambush to come out and seize the city, as both they and Jarchi observe, as well as a signal also to the army of Israel to prepare to turn and face about, and engage with the enemy; though they did not actually do this until they saw the smoke of the city, (Joshua 8:21) ;

for I will give it into your hand ;
of which the stretching out of his spear seems also to be a confirming sign to him, and which he kept stretched out until all the inhabitants of Ai were destroyed, (Joshua 8:26) ;

and Joshua stretched out the spear that [he had] in his hand toward
the city :
and therefore must turn himself towards it; and it is highly probable that at the same time there was a full stop of the army, and that they immediately turned or prepared to turn about.