The 3-4 forces your DL to stack up an OL and hold the line of scrimmage rather than attacking. Dareus, Kyle, and Carrington are all at their best when they're shooting gaps and disrupting. I'm happy to see Pettine is living up to his word about building a system around the athletes here, rather than forcing a system that may not be an optimal fit.
They will use both looks and IMO Carrington's ability to play both 4-3 DE and 3-4 DE is what allows them to disguise. Mario is listed as a DE there, and will be most of the time but look for him to line up as a LB so the offense doesn't know what they're looking at. The 3-4 will be used enough to create doubt, and to help defend the run when necessary. Otherwise it's one gap and let our athletes attack attack attack
Look for players to sometimes line up in positions we don't expect, to create confusion. Things like Mario lining up at MLB and Kiko at OLB on a given play- I'm using an example of something Pettine did in Jersey but who knows what he'll cook up for our team.
It's going to be fun watching a DC who makes an effort to confuse the opponent