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The 3-4 relies on stronger/bigger defensive linemen.
The 3-4 defense requires much different personnel than the base defenses that we have seen in the NFL in the past.
The first point of emphasis is acquiring larger defensive ends, who can open up pass-rushing lanes for outside linebackers. Without this, a defense may struggle getting to the quarterback on a consistent basis.
The second point of emphasis, and this is a biggie, is finding speedy edge rushers. These outside linebackers must be able to get to the outside and push past stronger offensive linemen.
Defenses who employ the 3-4 usually tend to focus primarily on building a strong front seven, while making the secondary stronger due to the specific scheme. This means that front offices of teams who employ this scheme tend to focus less on upgrading the secondary.
All of these things must occur if a team is going to have success utilizing this defense. If not, struggles will ensue.
Versatility and complex alignments are two big benefits for a 3-4 defense. It can cause mass confusion for offenses, which will lead to broken blocking assignments and more sacks. Couple this with the reliance on stunts and what I call "blind blitzes" and you have the perfect storm for success against more athletic offenses.
As we saw with Aldon Smith in San Francisco last season, you will see outside linebackers lined up as defensive ends. This causes issues for less athletic offensive tackles in pass protection and will enable the defense to put consistent pressure on the quarterback.
This type of defense can also mask issues in the secondary. Teams don't necessarily have to possess shutdown cornerbacks and can focus on stacking up on talent in the front seven.
Due to the fact that the NFL has fully entered the passing era, this defense has become more prominent than in the past. A lot of that has to do with being able to control passing offenses from the front seven, which can throw off the timing of the quarterbacks and receivers.
In order to be successful running this defense, teams need to have the right personnel all over the field. They need a run-stuffing nose tackle, which is a rarity in the NFL. In addition, they need to find pass-rushing outside linebackers. That can be troublesome, as most front offices attempt to move college prospects from a hands down defense end position to a hands-up outside linebacker position.
They also need beefier defensive ends to create pass-rushing lanes for linebackers.
Again, let's look at Aldon Smith in San Francisco. Once Justin Smith went down to injury, the young linebacker struggled a great deal getting to the quarterback.
It's fine and dandy to have athletic outside linebackers, but defenses that employ this scheme must be able to find defensive linemen who can fill gaps.
As we have seen in recent history, prospects attempting to make the transition from defense end to linebacker don't pan out a majority of the time.
If any of these positions are lacking, a defense will struggle a lot. This is only magnified if said defense doesn't have above-average personnel in the secondary, which would have normally been masked by a strong front seven.