Quote from Brass:
Scalp to concrete in a struggle would very likely produce cuts before concussion. Concussion depending on the severity and cuts because of any meaningful contact (which would very likely include an abrasion) with the concrete. If the slam was meaningful enough for Zimmerman to draw his gun and shoot, then that slam (with or without abrasion) would have caused bleeding. Zimmerman's story is not that Martin "pushed down on him." The word "slam" was mentioned along with "repeated."
If you've had any meaningful real-world training, then you know this already.
If the slam was meaningful enough then Zimmerman would be in a fucking coma or dead. You don't know what you're talking about... Getting hit in the back of the head puts your lights out really quickly... Your eyes glaze over and you go into a coma faster than you can imagine. It doesn't have to be a significant blow either. Hits to the back of the head are illegal because of this in MMA.
Also, concussions are caused by the fact that your brain hits the inside of your cranium... They happen because of violent shaking.
Concussion, from the Latin concutere ("to shake violently")[1] or the Latin concussus ("action of striking together"),[2] is the most common type of traumatic brain injury. The terms mild brain injury, mild traumatic brain injury (MTBI), mild head injury (MHI), minor head trauma, and concussion may be used interchangeably,[3][4] although the latter is often treated as a narrower category.[5] The term "concussion" has been used for centuries and is still commonly used in sports medicine, while "MTBI" is a technical term used more commonly nowadays in general medical contexts. Frequently defined as a head injury with a temporary loss of brain function, concussion can cause a variety of physical, cognitive, and emotional symptoms.
http://en.wikipedia.org/wiki/Concussion
Any swelling can instantly be life threatening.
I don't want to have to explain this to you again...