Quinn's scheme on defense isn't really about Blitz or even Sacks. I mean, they are nice if they come but that's not the objective. The objective is to force bad decisions. So, the front four are set up to force the OL back into the pocket. It doesn't have to be instant or immediate pressure, it just has to be progress. The LEO will take a wide 9 set a lot of times to get the corner and to basically flush the QB to one side of the field or the other. You want the QB to either run back into the rush of the other three DLs or to basically limit his field to only one half of the field. When you flush the QB, you are taking one side of the field away from him so the coverage is much easier. Most QBs don't throw across their bodies very well so it's an advantage to the Defense. This is why it's key to have a LB who can run and chase down the ball carrier because he will often be the pursuit guy that the LEO is flushing the QB towards. The other LBs need to be good in coverage and you need at least one who can play strong MLB run support role because there will be a run game that will need to be accounted for. After all, the NFL isn't quit to the point where they've just made the run completely illegal yet.
So this is the basic role for the front 7. In the secondary, you have a Strong Safety who can intimidate because you are going to see a lot of short to intermediate routes and you want the WRs to be worried about getting laid out. Also, you want that guy to be able to come up and help out in run support. This creates more opportunity for turnovers. So you have a QB basically being flushed, limiting his field of vision, you have CBs who can play bump and run, who can make plays on the ball, who can be disruptive. You have LBs who are dropping into short to intermediate coverage zones and you have a LB somewhere that is going to be scoping the QB or ball carrier and knifing through the blocking scheme to make plays or drag you down in pursuit.
The real star of this unit is the FS. That guy has to be a do it all dude. He has to be the guy that makes it all work. He has to be able to get to every area of the field, make plays on the ball to either break up the pass or to make a play on it to get the turn over. He has to be able to catch, he can't drop INTs. He must also be a good tackler because he's the last line of Defense that is going to be focusing coverage on half the field. So if you have somebody who can make a catch and break contain, he is going to have a lot of room to run if he can get to the far side of the field. That's when the FS comes into play. That guy must be able to walk down the ball carrier and make the play that saves 6. The FS has to be an all world type player. If you don't have that guy, this defense doesn't work.