It's all about the route, if the run is going outside and the blitz is coming from the other outside then the non-blitzing LBs will be shifting to cover and get caught shifting away from the run. That's how Dillon got that 25 yarder, outside of that one run though they held Dillon down (even with that run he only got 73 yards, but that TD run was huge, and a great display of why you shouldn't abandon the run even when the other team is shutting it down). Screens, crossing routes, and hooks are the way to kill a team that's blitzing, and the Pats are solid at all three.
Weis called a masterful game. You have to stay with the run even when it's not working just to keep the defense honest. Dillon was a back the Steelers couldn't ignore which opened up the passing game for Brady.