“ And you will rise up from the ambush, and take possession of the city. For YHWH your God will deliver it into your hand.”
At a signal from Joshua, made by raising his spear (verse 19) on some high point, for which they would be on the lookout, they would then move in and take possession of Ai. And they need not fear for YHWH would be with them and make them successful. For ‘YHWH your God' compare ‘YHWH the God of Israel' (Joshua 7:13; Joshua 7:19) a favourite title of Joshua's.