As soon as you hear the sound of marching in the tops of the balsam trees, move out to battle, because this will mean that God has gone out before you to strike the camp of the Philistines.”
God tells David to move when he hears marching in the tops of the balsam trees, because that sound means God has gone out before him.
This verse makes God's prior movement the signal for David's action.
8When the Philistines heard that David had been anointed king over all Israel, they all went in search of him; but David learned of this and went out to face them. 9Now the Philistines had come and raided the Valley of Rephaim. 10So David inquired of God, “Should I go up against the Philistines? Will You deliver them into my hand?” 11So David and his men went up to Baal-perazim, where he defeated the Philistines and said, “Like a bursting flood, God has burst out against my enemies by my hand.” So they called that place Baal-perazim. 12There the Philistines abandoned their gods, and David ordered that they be burned in the fire. 13Once again the Philistines raided the valley. 14So David again inquired of God, who answered him, “Do not march up after them, but circle around them and attack them in front of the balsam trees. 15As soon as you hear the sound of marching in the tops of the balsam trees, move out to battle, because this will mean that God has gone out before you to strike the camp of the Philistines.” 16So David did as God had commanded him, and they struck down the army of the Philistines all the way from Gibeon to Gezer. 17And David’s fame went out into every land, and the LORD caused all nations to fear him.