What to know when NFL helmets fly

The hit Arizona Cardinals quarterback Max Hall took from New Orleans Saints safety Malcolm Jenkins and two teammates Sunday separated Hall from his helmet -- and from the football.

The play was memorable for the violent collision and for tackle Levi Brown's recovery in the end zone for an Arizona touchdown. And, as NFL officiating director Carl Johnson explained on his weekly NFL Network show, the play also provided a good opportunity to more fully understand the rules.

The NFL added to its rulebook for 2010 the stipulation that a dead ball occurs "when a runner’s helmet comes completely off." But there are some potentially confusing variables.

The play remains live if the helmet comes off after the ball comes loose. That happened in Hall's case, allowing the Cardinals to recover the ball. Had the helmet come off before the ball was out, the play would have been dead. If the play had taken place in the final 2 minutes of a half or on fourth down, only Hall could have recovered.

The No. 1 lesson Sunday had nothing to do with the rulebook. Hall learned of the dangers associated with turning upfield and launching himself headfirst toward the goal line. Earlier Thursday, Hall's teammate, Larry Fitzgerald, promoted the quarterback for an online rookie award under the moniker Max "Learn To Slide" Hall.

Also: The NFL Network clip breaks down a play from the San Francisco 49ers' game against Philadelphia. In that case, the Eagles were prohibited from taking possession after pushing a 49ers player into the ball during a San Francisco punt return. The key, Johnson explained, was that the 49ers player was running down the field in a "passive" state as opposed to blocking.