The single biggest difference between rugby and American football tackles is that (typically) in rugby you put your head behind the the runner while in American

The single biggest difference between rugby and American football tackles is that (typically) in rugby you put your head behind the the runner while in American football you put it in front, i.e., across the body of the runner. The latter technique is only really possible without frequent facial injury when you're wearing a helmet and is used because it is more reliable (harder for the runner to run through). It also leads to more head, and especially head-to-head, contact.