Just looking at their make-up, four of the D linemen (Cox, Thornton, Logan, Geathers), are big and mobile, two (Curry, Square) are average size and mobile. All are young. The guy who's really surprised me is Logan; he's not just mobile, he's fast.

Those have all been plain from the jump, from the opener against Washington. DTs covering the tackles, NT in a 0-technique on the center, ILBs over the guards. Here we have Trent Cole (58) as the SLB. Plenty more on Cole in later segments.

With the season three quarters done, we have begun to see what Chip and Billy envision on defense, not that they have made much of a secret about it. Bigger, more versatile players. A base 2-gap 3-4. Strong tackling. Keep the play in front of you.

The one guy they've jettisoned is Isaac Sopoaga, who did not play particularly big nor was he mobile. He played hard but with limited efficacy and was flipped to New England for the equivalent of a late-round pick. With Logan ready to move up, it was the right move.

Here's an interesting play against the Giants that illustrates both Soap's shortcomings and Thornton's strengths. It also shows a significant change in the way Davis has his guys attack the O line. The first thing of note is that the DEs are no longer covering the tackles; they're covering the guards in a 3-technique. As we'll see as we go further along, Davis wants to attack the top of the pocket against the pass and defend the run from the inside out, as he does here.

Soap gets double-teamed. Unfortunately, he does what he did most of the season, which is stand up and both blockers are getting under him. Contrast that with Thornton and Cox, who both are lower than the guys assigned to block them. Both of the have engaged their men; Soap is barely out of his stance.

Soap's job is to make like at tree stump but he will, um, get washed away. Meanwhile, however, Thornton has won his battle and, while it looks as though he is going to step past the OG on the left...

...he throws the OG back and makes the tackle to his right. Soap is in that big melee. His job was to protect the ILBs so they could make the tackle; instead, he got driven back into them.

Chip Kelly said more than once that Thornton has been his best D lineman. Here's another example of why, also against the Giants.

Thornton shows over the inside shoulder of the OG. This looks like a good play for the Giants, with a lead blocker going into the B gap and Thornton sealed inside.

The center has slipped through to block Ryans.

Thornton does a beast move and sheds the OG.

And makes the stop.

So, what does Bennie Logan bring that Soap didn't? Well, he's cheaper of course, and younger, and much much faster. He also can stack and shed. Check out how he plays this inside zone run against Arizona. It's also a nice Billy Davis variation. Here Arizona is in a strong left formation and is bringing another player across, to make it stronger still. Logan as the NT is over the LG, and not the C. Thornton is in the weakside A gap and Cox is over the TE. Never saw this one before.

Thornton has half a step advantage to the playside and the OG encourages him to continue. This is the classic zone blocking problem for the D line: do you go to the hole or do you go to the ball? The answer is neither. You control your man until the runner commits. Thornton did a no-no here. However...

Even as the RB is cutting back behind Thornton, Logan is shedding his block.

And makes the tackle.