You don’t need to be a football expert or stat nerd to figure out why the 2018 Chiefs did not reach the Super Bowl. The league’s most explosive offense had carried one of its worst defenses throughout the year, and after losing the overtime coin toss in the AFC Championship, Kansas City had to lean on its weaker unit to give Patrick Mahomes a shot at taking down the Patriots.

As you know, that never happened. Tom Brady converted on third-and-10 three times during the game-winning drive before Rex Burkhead punched the ball into the end zone from a yard out to win the game.

Season over. One of the most prolific offensive seasons in NFL history had been wasted.

A change at defensive coordinator had to be made. It wasn’t just that Bob Sutton’s defense had failed to stop Tom Brady in a big spot — that happens a lot — but it was how the Patriots were able to convert those third-down plays that really exposed the flaws in his scheme. Sutton made Josh McDaniels’ job too easy: Without much schematic trickeration, the New England offensive coordinator was able to get one-on-one matchups for his best receivers thanks to Sutton’s basic man coverage calls.

On the first two third-down conversions, the middle was wide open for Julian Edelman, who just had to beat the man across from him. On the third and final conversion, Rob Gronkowski got a one-on-one matchup with a washed-up Eric Berry. It was just pitch-and-catch for Brady.

Two days later, Reid fired Sutton. Not necessarily because of that drive, but the drive did serve as a microcosm for the last few seasons of Sutton’s tenure. The Chiefs had finished 27th in defensive DVOA in 2018 and 30th in 2017 while the offense ranked near the top of the league.

Headed into the last two weeks of the 2019 season, the Chiefs defense now ranks 11th in defensive DVOA after holding the Broncos to a field goal in a dominant showing on Sunday. The turnaround is surprising given that there were no real changes made to the cornerback group and, after trading Dee Ford and releasing Justin Houston, the pass rush is arguably worse on paper. In fact, Kansas City’s pressure and sack rates have dropped off slightly from last season.

There may not have been wholesale changes to the defensive depth chart, but the changes the Chiefs did make have paid off. The big one was the hiring of Super Bowl-winning coordinator Steve Spagnuolo, who has employed a more complex coverage scheme that doesn’t put his corners in the same precarious spots Sutton’s scheme did in the key moments of the playoff loss to the Patriots. But a scheme is only as good as the players who are executing it; having smart, instinctive safeties was a prerequisite for Spagnuolo’s system to work. Enter free agent Tyrann Mathieu and rookie Juan Thornhill.

That trio has helped transform the backend of the Chiefs defense and it’s showing up in the numbers. After holding the Broncos to 199 net passing yards, Kansas City is up to sixth in pass defense DVOA after 15 weeks.

How have Spagnuolo, Mathieu and Thornhill helped turn things around? Let’s take a look…

We’ll start with a play against those damn Patriots. This is an early third down in Kansas City’s Week 14 win over New England. Now that Gronk is gone, the only Pats receivers who threaten a defense are Edelman and James White. Now, if Sutton was still calling the defense, McDaniels may have been able to get a favorable matchup in space for one of those two, as he did in overtime of the AFC title game. That wasn’t going to happen against Spagnuolo’s defense, though…

The Chiefs are taking a Belichickian approach here, and it’s something they did throughout this game and have done throughout the season. They have two sets of eyes on both Edelman and White, forcing Brady to go elsewhere with the pass. This defense isn’t going to let your best players beat them.

Against a mediocre Patriots receiving corps, Spagnuolo was able to aggressively bracket the Pats’ main threats, leaving no safeties deep. That’s obviously harder to do against other teams with more speed, but Spagnuolo has found creative ways to take away opponents’ biggest threats without leaving the deep part of the field exposed. One way he’s done it is by playing a Nick Saban favorite: Cover 7.

You can read more about Cover 7 here, but in short, it’s what’s known as a “man match” coverage that allows the corners and safeties to work in tandem against vertical routes to create leverage advantages for the defense. Let’s take a look at an example. For that, we’ll use Thornhill’s pick-6 on Derek Carr in Week 13.

Here’s the play…

Had the receiver to the to top of the screen gone inside instead of out, the safety to that side would have doubled him. But since he broke to the sideline, the corner could play with inside leverage and use the sideline as a help defender. An extra defender was not needed. That allowed the safety to bracket the receiver coming over from the other side of the field. Space is being constricted for both receivers.

At the bottom of the screen, you have the same concept with the receiver running wheel route. The Chiefs defender can play with inside leverage knowing he has the sideline constricting space for the route.

And then you have the bracket on Tyrell Williams. Thornhill and the corner are working in tandem: the former has inside leverage; the latter has outside leverage. Knowing he has help on the outside, Thornhill can jump the in-breaking route, which allows him to get the interception.

And based on the routes Spagnuolo expects to get based on film study, he can make different calls to change up how the defenders will leverage the routes. Here’s an example where the safety is playing outside while the underneath defender is playing inside, which is ideal against a corner route.

The Chiefs are still staying tight to routes, as they were in Sutton’s man-heavy scheme, but with the built-in help, they can also counter the popular tactics (picks, stacks, etc.) for defeating man coverage. Here the corner, who gets caught in traffic, passes off his receiver to the safety, forcing Carr to pull the ball down and ultimately take a sack.

The scheme is putting the players in a position to succeed, but they still need to make the play. Thornhill has been a ballhawk since his days at Virginia, so it’s no surprise that has translated to the pro level. And Mathieu is one of the more instinctive safeties in league history. He’s one of the rare defenders capable of carrying a receiver up the field while keeping his eyes on other potential threats, as he did on his interception of Carr in Week 13.

As we’re getting deeper into the season, the Chiefs secondary seems to be growing more comfortable in the scheme. After an uneven start to the season, the defense hasn’t allowed a positive passing EPA since Week 9. With the offense finally rounding into top form after getting healthy — it just put up over 400 yards and 27 points against a solid defense in a blizzard — and Spagnuolo’s defense playing like this, Kansas City has to be considered the top threat to the Ravens.

It looks like those two are headed for a matchup in the AFC title game. But before that happens, the Chiefs will likely have to get through the Patriots first. This time, though, they’ll have a defense they can count on.

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