Really, you got hit in the face and your mouthguard didn't stop you from getting a concussion? No shit man. Anyways, mouthguards do protect you from a concussion when your mouth gets slammed shut. Like I said before, 1 uppercut punch in a boxing ring without a mouthguard would knock somebody out cold, because it stops your jaw from railing your upper teeth. Think about it like 2 pool balls, when the moving one hits the one sitting still, the first ball stops and the second one picks up all of its kinetic energy. If you put rubber between the two, the effect would be much smaller. It is just like that, except your brain is in the middle of the second ball.