The hill country of Ephraim was to the northwest of Michmash.1 As the Philistine army in this area began to retreat and as they saw the army of Saul coming out to follow them, the other Israelites came out of their hiding places (likely in the woods) and joined the fighting.
22 Likewise, when all the men of Israel who had hidden themselves in the hill country of Ephraim heard that the Philistines were fleeing, they too followed hard after them in the battle.