Now don't get me wrong here, I'm a believer in fair play and not wanting to see any player getting injured.
But, if I was a DE and say it was mid way thur the first or second quarter, 3rd and 8 or more and I get a good jump off the ball, if I can't get a clear shot on they QB, say the OT is pushing me just past him, I'd reach out my arm and tap him in the back of his helment with a set of knuckles. Now I'd try to make look like a accident, and I might get flagged, and all you need to do is just a lite tap.
The effect of this is to screw his internal clock. Its like a good stiff jab will do to a boxer. For the rest of the game he'll be waiting for that loud pop, its very hard to re-group for that. Just say'in.