Adam Clarke on Joshua 8:18

Stretch out the spear - It is very probable that Joshua had a flag or ensign at the end of his spear, which might be easily seen at a considerable distance; and that the unfurling or waving of this was the sign agreed on between him and the ambush. (see Jos 8:13, and the preceding observations on Jos 8:1 (note), observation 6); and on seeing this flag or ensign unfurled, the men who lay in ambush arose and entered the city, making the fire previously agreed on. See Jos 8:8.