Reaves, arguably the NHL's best enforcer, caught the Red Wings defenseman in open ice while carrying the puck out. Hronek clearing doesn't see Reaves until the last second and pays for it.
After a lengthy discussion between the officials, Reaves was not given a penalty on the play, which was the right choice. Hronek was lower than Reaves, and it looked like the head contact was more arm than the shoulder.