But Yer Doin' It Wrong!
Traditional defensive wisdom goes something like this: Choose your front that you're going to stop the run with. From there, choose the coverage that you're going to run. Once you've done that, you can start to think more about techniques. I honestly think this is a very sound and responsible way to approach things and it really does work for the majority of defensive fronts/systems out there. If you're going to be a 4-4 or 3-3 team, you're limited in your coverage options because of how you've committed your players strictly by alignment. Such teams can run Cover 3, Man-Free, and 2-Robber relatively easily, again by alignment. If you're going to be a 4-3 team, you can run 2, 3, 4, man-free, 2 deep man under, you're almost unlimited in your possibilities. However, if you're running a 4-3 and you want to run Cover 3, then you have to work some stuff out, such as cloud or sky coverage, roll strong/weak, etc. This is slightly complex at times. If you're a 46 defense, you'd better be running Man-Free or some variation of Cover 3, such as 3 deep 3 under fire zone.
This works because of how we have set up our understandings of force and contain, pursuit and coverage. If you're playing defense, you need to have players assigned to forcing everything back inside, period. That said, who can perform those roles depends greatly on where they're aligned. A Free Safety cannot align at 12yds deep in the Strong A gap and be responsible for weak force, unless he also wears a cape and has a big red "S" on his chest. I hope I don't need to make more examples of this.
Because we choose our front first, we are dedicating a certain number of people to certain alignments, thereby limiting the number of possible assignments. If we commit 8 players to the box with our front, we cannot have 2 safeties. If we only have 7 in the box, then we must have 2 safeties. Recognizing this allows us to have a better understanding of how coverages fit into defensive structures.
And Here's Where I Contradict Myself…
I really, truly believe that for a person implementing a 3-4 scheme, choosing the coverage first is crucial. The 3-4 has a lot of moving parts, more so than just about any other defense, and often has changing responsibilities with regards to force, contain, spill, all those terms we love to use to define good defense. The difference between the 3-4 and other defenses, in my experience, is that the 3-4 has the interesting feature that the front and the coverage are intertwined. If you want to run a certain coverage, you need to do certain things with your front. There is a minor assumption that is working behind all of this: you want to rush at least 4. If you don't mind rushing 3 and dropping 8, well, no biggie. But if you're going to rush 4 in the 3-4, you need to marry the front and the coverage. You have to make a conscious decision about what you're doing.
The 3-4 is a seven man front to start. The actual front alignment varies quite a bit, with some teams preferring a 4-0-4 head up approach with slanting and stunting, and others preferring an 'under' front variation (9-5-1-3-5), and yet others running a 3-0-3 double eagle front. That's fairly irrelevant at this exact moment. What is important is how you're going to run your coverage, specifically what your base is. It comes down to this: are you going to be an even coverage base or an odd coverage base? Are you going to run Cover 3, Cover 1 (Man Free) and Cover 9 (3x3 fire zone) or are you going to run Cover 2, 4, and 6 (¼ ¼ ½)? Answering this question is the biggest step towards developing a common sense, fundamentally sound 3-4 scheme.
If you're going to run Cover 3, then you need to blitz someone (an LB most likely) and probably replace them with a DB. Who the someone is doesn't matter, you need to blitz someone to send 4 and drop 7. If you blitz an OLB, then the safety on that side should replace him in coverage, presumably with the Curl/Flat responsibility. If you blitz an ILB, probably same solution, except now it's Hook/Curl. You can just straight up send a safety and everyone else drops, if you really want.
If you're going to run Cover 2, then you need to blitz someone away from the passing strength or wide side of the field. Now you don't want to blitz your corners in C.2, they have a pretty important responsibility, so that's out. Similarly, you want to keep your safeties deep, so they can't blitz. Therefore, it's one of your ILBs. The reason why I say away from passing strength is that the three interior drops in Cover 2 usually go Hook/Curl, Middle Hole, Hook/Curl. Because of that, you generally want more people dropping to the passing strength because you want to have numbers to the passing strength.
I don't think I get it…
No worries, it's a complicated concept and one that is unusual. The 3-4 is a complicated and unusual defense these days and I really believe that if you sit down and marinate on what I'm talking about, you'll notice there's a certain logic within that makes it sort of an 'Ah-ha!' realization. I stumbled on the importance of this while implementing our 3-4 two years ago. I was reading a thread on Huey where someone mentioned the approach mentioned in my intro and I realized that it didn't work that way for the 3-4. After that, I began to think on it more and more and I feel like I've got a good grip on the mechanics of it all.
For more reading, I really recommend hitting up my scribd account and looking at some of the playbooks there. There's a neat synergy between the front and the coverage and how it all just… works.