I spoke at length with mister McCann hisself about it (he's a table holder at a local gun show). The idea is that the full pressure is applied to the bullet, but then vented before it acts on the action? Full velocity, but less wear? Sounds like a good idea.
I do like the idea of using the large port to bleed enough gas to turn the gun into a straight-pull bolt action - after spending about four hours preparing 100 cases for loading, I'm in no mood to lose even one!