The main danger in getting hit in the back of the head is the brain stem is right there which is the control board for the central nervous system. The occipital lobe is also back there which is why G-Man is now blind from being hit in the back of the head.


Since the stem is where the brain is anchored it stands to reason the brain won't slosh around as much if you hit it from the back but it'll cause more damage as the back of the head is the most vulnerable part. The frontal lobe controls emotions and memory retention which is important but not as important as telling your heart to beat and your lungs to take in air which is what the brain stem does.