The 2010 season was a very significant one in the NFL, because it signaled a shift in league policy about how it would enforce its protections of defenseless players, and preceded a series of new rules, which continue to be refined and added to, furthering those protections.
And James Harrison was the poster boy for exactly the type of player that the league was trying to get rid of, after he sent two members of the Cleveland Browns out of the game with concussions. One hit, against his former college teammate Joshua Cribbs, was technically legal. The other, on wide receiver Mohamed Massaquoi, was not.