DENVER — Last year, the key to beating the Denver Broncos was simple: be one-dimensional on offense.

This year, the only way to beat the Broncos is by being one-dimensional on offense.

The difference: it’s the exact opposite dimension.

In 2016, Denver’s vaunted “No Fly Zone”—the top pass defense in the league for the second-consecutive season—forced opposing teams to run the ball. Last year, teams avoided throwing the ball against the Broncos at all costs.

Not only did Denver have the best pass defense in the league for two straight seasons, their run defense was like Swiss cheese—full of gaping holes. The run defense finished the year as the fifth-worst in the league, giving up 130.3 yards on the ground with teams running the ball an average of 30 times per game, third-most in the league. If teams wanted to run the ball, the odds were highly in their favor.

2017, however, has been a completely different story. After a 16-10 victory over the Oakland Raiders on Sunday, Denver’s run defense finished with the best start in franchise history, only giving up 203 total rushing yards through the team’s first four games—an average of 50.75 yards per game on the ground, nearly a third less than last year.

“We just emphasize stopping the run, no matter who the running back is,” Broncos safety Will Parks said emphatically following the team’s third win of the season. “We emphasize playing the run. We’ve got the ‘No Fly Zone,’ that’s why they call us that. It’s just stop the run.”

With four Pro Bowl running backs—Melvin Gordon, Ezekiel Elliot, LeSean McCoy and Marshawn Lynch—combining for 95 yards against the Broncos in the first four weeks, or an average of just under 25 yards per game, teams have been forced to become one-dimensional in order to find success against the Broncos’ defense.

That means they’ve been forced to go after the *gulp* weaker part of the defense. Unfortunately for opposing offenses, that “weakness” is filled with eight Pro Bowls between cornerbacks Aqib Talib (four) and Chris Harris Jr. (three) and safety Darian Stewart (one).

“I harp on it all the time, when you talk about stopping the run. If you can make a team one-dimensional and you put them in situations where the have to pass the ball, it makes us so much more effective in the pass game, especially with our rush upfront,” second-year safety Justin Simmons explained. “You can’t ask for more as a secondary for that front four we have so stopping the run was huge.”

Now that teams have been forced to throw the ball against the Broncos early and often in games, opponents are finding a bit more success than they have the past two years.

“Our pass coverage is pretty tight. But we’ve given up too many explosive passes that have led to points,” Broncos’ head coach Vance Joseph said Monday afternoon after examining the film from the game tape on Sunday. “I think it’s more focus in the details. They are plays that we’ve covered in practice that hurt us. We got to be better in our focus in the details.”

Against the Raiders, Denver gave up a 67-yard touchdown through the air as well as passes of 28, 22 and 20 yards. In Oakland’s 31 other pass attempts, Denver gave up just 93 yards.

The past two seasons, in which Denver has found tremendous success against the pass, they’ve primarily been a man-coverage team. This year, while still primarily man, they’ve played more zone coverage. The learning curve of zone could be a major reason in Denver’s few lapses in coverage.

“When you are a zone team, you have to be really really focused on your keys. When you are a man team, you just play your man. That’s easy to do from a mental standpoint and hard physically,” Joseph explained with great passion as he leaned on his background as a defensive backs coach. “We are trying to give these guys a break and not play so much man, but we have to train it, and they’ve got to continue to play with more focus and detail. Zone takes focus and detail.”

Although initially, it seems as if Denver’s pass defense has taken a step back—as they are giving up 210 yards through the air in 2017 compared with the 185.8 they gave up last year—a closer look shows that’s not necessarily the case.

In the first three quarters—when Denver had a lead in all but one game—the Broncos gave up an average of 123.75 yards through the air, or a pace of 165 per game. However, in the fourth quarter—when teams have been forced to throw the ball since they are losing and Denver is playing in more of a preventative defense—they’ve given up an average of 86.25 yards.

Thus, when Denver isn’t in a prevent-type defense, trying to hold onto a game, they are still extremely difficult to pass on. However, since teams can’t run the ball on Denver, they’ll be forced to attack the “No Fly Zone.”

On Sunday against the Raiders, that’s just what happened. Rushing for 24 total yards on the day for an abysmal 1.6 yards per rush, the Raiders had no other choice but to attack Denver’s secondary. Through the first three quarters, Denver had given up a net total of 139 passing yards to Derek Carr and E.J. Manuel—an average of 46 yards per quarter.

However, in the fourth quarter, down 16-7, the Raiders had no other choice but to pass. In the final period, they accumulated 91 yards through the air—far more than their 46 average the first three quarters—but when Denver’s secondary needed to tighten up and not play prevent defense, they rose to the occasion.

With 1:46 left in the game, Simmons came up with the game-clinching interception to end all hopes of Oakland’s comeback.

“One of our young bulls stepped up like a vet in the clutch. High pointed the ball; you cannot do much better than that. So that is a big-time play by ‘J’ [Simmons],” Talib said like a proud father. “It is our preparation. We have great confidence in ourselves. We do not second-guess ourselves in those kind of situations. ‘J’ definitely did not second-guess himself. He went up and made a great play.”

If Denver’s run defense maintains their dominance they’ve displayed over the first quarter of the season, opposing teams will have no choice but to attack the leagues best secondary over the past two seasons.

“The passing game is where you score points, and everyone understands that. You can give up 150 yards rushing and be in a football game,” Joseph said. “The pass game is where you make your chunk yards and where you score points.”

In a passing league, especially with Denver’s run defense playing the way it has to start the season, opposing teams may just be forced to beat the Broncos through the air. Recent history would say ‘good luck.’