Under the change, a player will be penalized 15 yards and potentially ejected any time he lowers his head to initiate and make contact with his helmet against an opponent. It will apply to tacklers, ball carriers and even linemen, and it will take the place of a previous rule that limited the penalty to contact with the crown of the helmet.

The NCAA's targeting rule penalizes players only when they hit opponents who are in a defenseless position. It calls for mandatory ejections, but the NFL's competition committee has not yet addressed how ejections would be adjudicated, according to chairman Rich McKay. There is little doubt, however, that the NFL is determined to aggressively address a 2017 season that included 291 concussions, its highest total on record, and a severe spine injury to Pittsburgh Steelers linebacker Ryan Shazier on a play that would fall under the new rule.

"It just seems that players at every level are getting more comfortable playing with their helmets as a weapon rather than a protective device," McKay said. "Therefore, we need a rule that is broad and puts that in context, and that's what we think this does."

Players, coaches and fans were left guessing on how the rule will impact the game. NFL Players Association president Eric Winston took to Twitter to share his thoughts.

According to NFL research, nearly one out of every two helmet-to-helmet hits caused a concussion in 2017. That's up from a ratio of one out of every three in 2015. NFL chief medical officer Allen Sills said in February that the current concussion data had sparked a "call to action," and on Tuesday he said this rule would be a key part of reducing head injuries in 2018.

"We spoke previously this year of having an all-time high of concussions," Sills said. "And we said that wasn't acceptable, and that we would respond to this, and this was part of the response. This is a very key component of the injury-reduction strategy on how we can reduce concussions immediately."

The competition committee initially planned to make lowering the helmet a 2018 point of emphasis rather than a rule change, McKay said. But after a leaguewide discussion Tuesday, owners instructed McKay to convert it to language that could be added to the rule book immediately. The league called a late-afternoon news conference and acknowledged that some parts of the rule still must be fleshed out.

At the top of the list is how to merge a long-standing league ethos against two issues: wide-ranging ejections of players; and using replay to review what are considered subjective calls by officials. McKay said the league is trying to effect a change in "behavior" and thus likely needs the weight of an ejection to communicate its sincerity. And given the potential impact on a game, a replay review is almost certain to be necessary to ensure proper enforcement.

"If you put replay behind [officials]," McKay said, "then I think that you do have the opportunity to feel more comfortable in ejecting them. But in this rule, we just need to do a little work to understand a little bit more how the mechanisms will work. But I think we do feel comfortable that if there is an ejection that replay would probably play a part of it."

The NFL will spend the next two months further developing the rule and likely will alter it to address replay and ejections at its May 21-23 meetings in Atlanta.