Hit from behind into the boards.
The only way it isn't is if the refs just don't want to boot a guy in this situation.
And I really don't think the refs had a whole lot of latitude on it. At least a couple years back, this was practically a mandatory call.
If the refs are feeling really bad about it, I wouldn't be shocked if they called a soft penalty on ND to even things up a bit.
I won't be surprised if there's no makeup call.