While watching a highlight video on Youtube of various hockey hits, I noticed something that hockey players do nearly indiscriminately that NFL players don't. Hockey players lead with their shoulders, and the results can be just as violent as leading with the head, granted many of the hits in the video I've embedded here would be considered illegal by NFL standards.
If you already haven't, watch the video above. Hockey players are able to lay massive hits on opponents while maintaining their balance, on ice. NFL players lay hits of similar caliber, but many times, both players involved in the hit find themselves on the ground.
Leading with the shoulder has grown naturally in the NHL for a couple of reasons. One reason may be the lack of a facemask, but a more likely reason is the balance hitting with the shoulder affords players. It allows a skater to hit another player and keep going while the victim picks himself off the ground.
As we head into the NFL preseason, expect to see a new round of players launching themselves head first at opponents, and expect yellow flags to follow. As the most (or second most violent depending on who you ask) violent major American sport, hockey hits can translate to some extent on the football field, and I contend that NFL players can learn a lot about effective hits from watching a little bit of hockey in the offseason.