Charles Ellicott Commentary Psalms 18:40

"Thou hast also made mine enemies turn their backs unto me, That I might cut off them that hate me." — Psalms 18:40 (ASV)

You have also given. —Literally, and as to my enemies, you gave to me the back, which either means “turned to flight so that only their backs were visible” (Jeremiah 18:17 and Psalms 21:12), or alludes to the common symbolism of defeat—trampling on an enemy’s neck.