When mine enemies are turned back; or, because mine enemies are turned back ('Speaker's Commentary'); i.e. made to retreat, repulsed, driven before me in hasty flight. They shall fall and perish at thy presence; or, they stumble and perish, etc.
The psalmist represents the enemy, poetically, "as if they had been thrown to the ground by the glance of God's fiery countenance" (Hengstenberg).