Let them be as chaff before the wind (comp. Psalms 1:4; Isaiah 17:13; Isaiah 29:5; Hosea 13:3). Chaff is the type of whatever is light, vain, futile, and worthless; chaff driven before the wind represents the confused rout of a beaten army flying without any resistance before an enemy.
And let the angel of the Lord chase them; rather, smite them. The angel of the Lord, who protects the righteous (Psalms 34:7), is called on to complete the discomfiture of the wicked ones, who are David's enemies.