Why if you hit somebody from behind his head seems to snap backward?

When someone is hit from behind, their head snaps backward due to the sudden acceleration of the body and the inertia of the head. The body moves forward while the head, momentarily lagging behind due to its inertia, is pulled back by the force of the impact. This motion can cause strain on the neck and potentially lead to injuries such as whiplash. The snapping motion is a result of the head's attempt to catch up with the rapidly moving body.