In the Flyers-Blues game today, there was a scrum and two players from each side were called for roughing (two minutes each). When play resumed, the teams were playing at full strength.

However, it usually seems that, if there's only one pair of matching rouging penalties, that teams play four-on-four. So how is it determined how much manpower teams have after matching minors? I know matching fighting majors don't change manpower, and I think that any other matching minors than a rough (i.e. a high stick and a holding) results in four-on-four play. I've tried to figure out if it has to do with when the penalties happen (during play or after the whistle has blown) and I can't figure it out. Any insight?