When the human head hits the playing surface of an NHL rink it hits 3/4ths of an inch of ice that sits atop a concrete slab. We most frequently see heads hit the ice during hockey fights, which is why the league decided to outlaw the intentional removal of one's own helmet before or during a fight before the 2013-14 season. This rule was one of the NHL's more recent attempts to limit the amount of concussions we see during a hockey season.
Of course, not every hockey fight results in a players head slamming down to the ice surface.