Therefore shalt thou make them turn their back; literally, their neck (comp. Is. Psalms 18:40). The meaning is simply, "Thou shalt put them to flight." When thou shalt make ready thine arrows upon thy strings against the face of them.
The Authorized Version, by supplying "when" and "thine arrows," expresses what the psalmist has left to the intelligence of the reader. The psalmist says, "Thou shalt put them to flight; thou shalt make ready upon thy strings against the face of them, no doubt meaning that the discharge of arrows would produce the hasty flight, but not saying it.