There are so many interchangeable parts in how you run a defense, but it mainly comes down to gap responsibilities and what you want your specific players to have to do.
I'm surprised the article refers to a 4-3 Under as a Tampa-2 because in reality, it's really a misdiagnosed 3-4 most of the time.
We're much better equipped to run a 4-3 Under than we are a 3-4 unless we're just wanting to rush 6.
In the 4-3 Under you have your NT playing 1 technique between the C and strong side G. Curtis Porter would play this. You need the NT to command a double team regardless of 3-4 or 4-3 Under.
In the Under your 3-technique will line up between the C and other G and is generally going to get a one-on-one if your NT causes double and it's hard to double this guy anyway due to where he lines up. Olsen Pierre would man this spot.
What the Under does that the 3-4 doesn't is protects your ILB's from having to take on G's. In the 3-4 your Under DT is now a DE playing either the 4 or 5 technique against a T. 4 is head up, or a 5 is on the outside shoulder. We don't have the LB's to be taking on G's, so I'd rather protect them personally.
In the Under, you still need a strong side DE who is strong enough to handle the run and set the edge even against a TE chip as well. We have a decent prospect in Chickillo and some hold out hope for Jelani Hamilton. In the 3-4, it's fairly similar here as well, as your DE's have big outside assignments against T's.
On the other side in a 4-3 Under though is where you put your best pass rusher. He has his hand in the dirt and is a rusher almost exclusively. For the Seahawks, this player rushed the passer 96% of the time, rather than dropping into coverage. So while he technically plays DE, you can use Bond or AQM or McCord here and be fine.
Another difference is the 4-3 Under still allows you to line up your SAM on the outside shoulder of the TE and get a jam on him, or maintain the edge. This will be Figueroa most likely, and he has the athleticism to stack and shed or rush the passer or cover the flats. This player rushed around 60% of the time last year for Seahawks.
Gaines is going to have to be a thumper (or Kirby) in the middle and be a sure tackler for this to work, but you don't have to send him down the middle of the seam constantly like the Tampa-2 does.
Your WILL is going to get things funneled to him and he's going to have to run and chase like a madman to make up for your DE (who would be an OLB in a 3-4) rushing the passer on nearly every play. He has to cover flats, fill the run hard, cover backs and TE's when his assignment calls for it and make plays. If only we had a mad man LB'er with run and chase skills like, say, Denzel Perryman.
For me, with our personnel, we should run the 4-3 Under quite a bit to protect the ILB's from having to take on blocks if we played a 3-4, but again, the responsibilities of either mainly come down to gaps mainly. Most teams that run a 3-4 are running a 5-2 in reality with placement of their personnel.
Good conversation...enjoyed the thread.