A coach doesn't tell you to drop your head. But if you drop your shoulders...the head and neck attached to it comes along with it.
When you're runnings drills through the machine where you cover the ball with both hands, you're leaning shoulder forward with the head being in front.
You can't police that.
right and its your facemask and hairline that makes contact with the other players shoulder/helmet....which is still legal.
the rule is stupid because that instance rarely ever happens where a player aggressively turns his head downward as a bulldozer(which is crazy dangerous to)....but I doubt this really ever gets called.