I just saw the final wording, which is quite surprising:
“It is a foul if a player lowers his head to initiate and make contact with his helmet against an opponent."
That's it, in it's entirety.
Nothing about "outside the tackle box", which is what I read earlier. Omitting that aspect of the rule is a major, major change. If that is in fact the way they are going to rule, there's potential for a flag on every run down. It could be a defender drawing the flag, but it could also be a lead fullback/H-back or the runner himself getting low and initiating such contact. It could be linemen knocking heads when both go low. In fact, on a run between the tackles, as the game has always been played, somebody making contact with somebody else with his helmet is probably the rule rather than the exception.
So now everybody has to play high/heads up in what was the highest impact part of the game? Run blocking becomes like pass blocking patty cake? Expect more leg injuries.
Upright runners with a good jump cut and stiff arm might be happy.
What will McCarthy do about pad level?