There are many ways in which the Penguins system is built to insulate their defense. Having speedy forwards allows for pushing from the D and making sure that if the D misses coverage, the forwards can cover. This isn't the same level as say a Sutter system where everyone is covered at all times, but the forwards are fast enough to get to the point where they can interrupt/obstruct out number attacks. Many times the Penguins don't actually stop rush chance shots, they simply make them very difficult. Once the rush chance gets quelled, they can pick up the puck in their D zone and fling it out to a streaking forward. It helps having an amazing tandem of goalies as well.
However, I believe a stronger possession team could give them trouble.