‘ And when the men of Ai looked behind them, they saw, and, behold, the smoke of the city ascended up to heaven, and they had no power (literally ‘hands') to flee this way or that way, and the people who fled into the wilderness turned back on the pursuers.'
The word for ‘hands' is the same as in verse 18. Joshua stretched out his hand, the city was given into his hand, while Ai had no hands like Joshua had. They had no special power to call on.
Suddenly the battle changed. The men of Ai soon became aware that their city had been sacked, for the smoke of the city ascended up to heaven (compare Judges 20:40), and they looked back and saw it. The city was as one great burnt offering to YHWH. They had no help to look to, nowhere to hide, no city to fall back to, and when the ‘terrified' people they were chasing suddenly turned round and did not appear terrified at all, it had suddenly all become a nightmare.