I've just had a look at the NFL coaching video that will explain how to apply the rule to the officials. For your perusal I've transcribed the main features here relating to this particular rule. I've tried to be as accurate as possible with regard to the language. This is pretty much word for word what is said.
Where can it occur:
Forcible blow must be delivered outside of the tackle box... tackle to tackle from the outside frame of the tackle. This goes all the way back to the offensive end line extends 3 yards beyond the line of scrimmage. Rule does not apply within that area.
Has to have 3 parts:
a) Player must square up or line up his opponent
b) Player lowers his head
c) Player delivers a forcible blow with the top crown of the helmet
All three parts of the process must be in place for it to be a foul.
2 to 3 sets of eyes looking at play, at least 2 officials will confer.
Rule also applies to the defense.
Enforcement: Spot foul 15 yards and replay of down.
If player is not lining up the opposite player and is lowering his head to protect himself and the ball this is legal. The Running back can still lower his head to protect himself and absorb the blow. Blow must be delivered with the crown of the helmet, if he lowers his head and delivers blow with shoulder this is not a foul.
The foul has to be clearly outside the tackle box. Inside the tackles at the goal line it is all going to be legal. It also has to be outside the tackles to be applied at the goal line.
Having seen examples on the video, I can really see what they are talking about now and why it would be dangerous. I think it's important to note that it specifically says that a player can lower their head to protect themselves and absorb the blow. It would seem that for this to be a foul the player must line up the opponent and use his helmet with intention to land a blow.
Anyway I am sure that greater minds than mine will be able to have a look and tell me what they think. Pretty sure I got the video from the NFL website, will try and check out where or see if I can post it if wanted.
Hope this is of interest, it kind of clarified things for me as well.