I think of it as the top, or point of helmet. I think of it as leading with the helmet to make the tackle.

There must be more to the rule. There is no way that either hit could be interpreted by a sober, sane person as one player targeting another's head with the crown of the helmet. In Nacua's case, it is obvious to a billion people in China that he was consciously attempting to avoid that scenario, thus not targeting. Incidental contact with the helmets does not meet the criteria of targeting. This is a gross misinterpretation of the rule. These rogue refs/replay officials need to be reigned in.