system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What happens behind the scenes when a person is clubbed and blacks out?

How is it possible for a person to black out immediately after being hit?
gpt
Movies and TV show this  pretty unrealistically. It's a lot easier to kill a person with a head blow than to just knock them out. This is especially true of hitting the back of their head, where the section of brain matter that controls a lot of autonomic systems happens to sit.

Unconsciousness is usually caused when your brain stops receiving enough blood to operate(often by the brain swelling from an impact, think of it like a bruise that lifts up, that presses on your brain and cuts off blood flow.) Obviously, that's not a good thing. Blood is food and your brain needs it.

Even when people are classically 'knocked out' they usually have a pretty severe concussion and have to be kept under observation for days or weeks following such a blow as it is considered head trauma, and severe head trauma if the person was knocked out for more than 10-20 minutes. Starving your brain for blood can cause long-term damage after a very short time. Boxers usually develop a lot of these problems due to this, including memory loss, confusion, fatigue, ect.

Now, that being said... there is an odd nerve near the jaw that can be hit which causes a stun knockout. You see this in boxing occasionally, but the knockout from those hits usually only lasts a few seconds, not hours like they have in movies, and the hit has to hit the jaw, not the head.

(I do a lot of weird research for writing)