Getty Images

The Denver Broncos started off the season looking like a disaster. They went 1-4 in their first five games, with the only win being a game where they had trailed by double digits against the to-that-point winless Chicago Bears. Worse yet, Denver gave up 28 or more points to every team but the hapless Las Vegas Raiders, and got beat up for SEVENTY points and SEVEN HUNDRED AND TWENTY-SIX yards by the Miami Dolphins.

During that five-game span, the Broncos were the worst defense in the NFL by a mile, and they were pretty close to the being the worst against both the run and the pass, as well. But a funny thing has happened since then: Denver has won three of its last four games, and didn't allow more than 22 points in any of them. After forcing just five turnovers through the season's first five contests, the Broncos have come away with 11 in their last four games, and are working on back-to-back games with four or more. By just about every measurement, it has been a stunningly dramatic turnaround.

By both FTN's DVOA (Defense-adjusted Value Over Average) and Tru Media's EPA/play, they are the most improved defense in the league during this more recent stretch, and it's not particularly close.

Defense (Rank)Wk 1-5 DVOAWk 6-10 DVOADiffWk 1-5 EPA/PlyWk 6-10 EPA/PlyDiff
Run12.8% (31)-2.1% (24)-14.9% (4)0.10 (31)-0.03 (20)-0.13 (7)
Pass63.5% (32)-8.8% (7)-72.3% (1)0.43 (32)-0.04 (3)-0.47 (1)
Total38.4% (32)-4.0% (9)-42.4% (1)0.20 (32)-0.06 (11)-0.26 (1)

When we see massive improvement in-season like this, it's important to look for the root causes.

When it comes to the run game, the answer seems simple. Not having the ridiculous Dolphins game included props up the numbers quite a bit. Miami ran for an outrageous 350 yards on 43 carries -- an average of 8.1 a pop. The Broncos missed about a zillion tackles in that game, while also allowing several runs where the back when untouched all the way into the end zone. Including all of that in a relatively small, five-game sample can throw the numbers out of whack. 

Since that time, Denver has basically just been a run-of-the-mill, below-average run defense. The Broncos have actually allowed an even higher explosive run rate (11.2% of carries vs. 10.8% since Week 6) than they did through Week 5. But they've yielded about a half-yard less per carry both before and after contact, and a significant part of that can be traced to the Dolphins debacle.

It's much more interesting to dig into the pass defense improvement, since the team has gone from worst in the league there to borderline top-10 over the past several games.

Some of it is personnel. Earlier this season, the Broncos were using Damarri Mathis as their No. 2 cornerback across from Patrick Surtain II. Fabian Moreau took over the job in Week 7. Prior to the switch, Mathis allowed 27 of 37 passes thrown his way to be completed, for 361 yards and four touchdowns, checking in 122nd in out of 124 qualified corners in Pro Football Focus' coverage grades. Moreau has yielded 12 completions on 19 attempts for 110 yards and an interception while ranking 22nd out of 113 qualifiers in coverage grade. That's a significant upgrade. 

But defensive coordinator Vance Joseph has also made some changes to the way the defense is structured and how he deploys the personnel within it. 

According to Tru Media, the Broncos are playing significantly less often out of their base defensive personnel (21.7% of snaps compared with 35.8% earlier in the season) and playing much more nickel (69.2% compared with 54.2%). Having that extra defensive back on the field rather than a lineman or linebacker has helped the pass defense in several ways, one of which is that it has allowed Joseph to blitz slightly less often and keep more players in coverage. That's helped the Broncos cut down on explosive plays. They allowed 10.2% of opponent pass attempts to result in a gain of 20 or more yards through the first five weeks of the season, but since then that rate has dropped significantly, down to 6.6%.

The Broncos are also playing more zone coverage lately, getting there on 77.5% of their coverage snaps as opposed to 66.4% early on. Asking those weaker cover corners to play so much Cover-1 earlier in the year contributed to the ridiculous rate of explosive plays. 

Crucially, the Broncos have also gotten pressure on the opposing quarterback more often. Early in the year, they were at a significantly below-average 28.4% pressure rate, per Tru Media. In the more recent games, they're slightly above-average, picking up a sack, hit, or hurry on 36.4% of opponent dropbacks. The biggest leap his come from Nik Bonitto. He had just 90 opportunities to rush the passer through the first five games, and he pressure 15.6% of the time. In one fewer game, Bonitto has gotten 86 pass-rush opportunities and turned in a 17.4% pressure rate. The return of Baron Browning has also been huge. He didn't play in any of the first six games of the year, and he's picked up 10 pressures on just 59 pass-rush snaps since his return. Free-agent signing Zach Allen has also picked things up, going from a 7.4% pressure rate through five weeks to 14.6% over the past four games.

More defensive backs, more pressure, fewer explosives... these are all things that help a pass defense. And the Broncos are getting all of them at the moment. Ahead of a Sunday Night Football date with Josh Dobbs and the Minnesota Vikings (8:20 p.m., NBC) this is a defense that is moving in the right direction, and could be feisty over the second half of the year.