Coverages are the backbone of defensive structure, oddly enough, because they determine how and where the front defenders match their gaps and force the ball to go in pursuit. You cannot reasonably ask a cornerback to have deep coverage responsibility down the sideline while simultaneously containing against the perimeter run game.
How You Cover Determines How You Stop The Run
Knowing who has the deep coverage responsibility allows the rest of the defense to formulate who is responsible for protecting the perimeter. You often hear terms like "Backer Force" or "Safety Force" - the Force player is simply the contain man against the run. He must play with his outside arm free to keep outside leverage on the ball and either make the tackle or turn the ballcarrier or "Force" the back inside.
Cover 2 defenses have the cornerbacks responsible for an underneath zone, so it allows them to play as force defenders. Hence, defenses that run a lot of Cover 2 tend to try and play inside/out, "spilling" the ballcarrier and making him run laterally towards the sideline. One-high defenses (Cover 3 and Cover 1) have the corners vacating with deep pass responsibility, leaving an outside linebacker and one of the safeties responsible as the Force players.
Image via TomahawkNation.com
The third major distinction between one-high and two-high defenses is how they defend the number of offensive gaps in the box, or how the defense "fits" against the run. A base offensive formation (12 or 21 personnel) presents eight gaps for the defense to defend - nine if the quarterback is a running threat, since the runningback can be used as a lead blocker (and a lead blocker is just a movable gap).
While the Cover 2 defense gives the corners run responsibility, they're still detached from the offensive formation and aren't a factor against the inside run. With only seven defenders to occupy eight gaps, the defense is forced to rely on certain players to be responsible for two-gaps in the defensive front. Most of the philosophical differences amongst defenses rotates around who in the front two-gaps and how that affects the run fit for the remaining inside defenders.
In a two-high, one-gap defense like the Jaguars ran last year, the linebackers must adjust their flow based on the ballcarrier's path to account for the cutback.
By bringing a safety down into the box, the run fit is altered. The backside defender (the outside linebacker or safety that the play goes away from) will automatically be designated to defend the cutback, allowing the other interior pursuit players to fast-flow to the ball without worry).
Cover 3 and It's Variations
The basics of Cover 3 as a coverage are simple enough: three deep defenders split the field into thirds, and four defenders split the six underneath zones. Two Curl/Flat defenders are responsible for the area just outside the hashes at first, and then expanding to the sideline. The Hook/Curl defenders are responsible for re-routing vertical threats up the seam and then defending against inside breaking routes around the hash marks. Everything is predicated on being able to defend the middle of the field with inside/out leverage.
In the Cover 3, your Force and Cutback players are almost always going to be your Curl/Flat defenders - that's how the coverage and run fits marry, and if you can get that concept, you'll understand almost every variation of this defense.
The above picture (and pictures I drew up from the Jaguars/Vikings game) are the basic form of Cover 3, where one of the safeties comes into the box and picks up Curl/Flat responsibility with the opposite outside linebacker (weakside in this case) picking up the other Curl/Flat. A defense will name it using some variation of a word starting with "S" (Cover 3 "Sky" or Cover 3 "Slam"). Against the run, their Force/Cutback duties will simply be determined by which way the ball flows.
Between the Hook/Curl zone and defending against the perimeter run, the Force player is responsible for covering a lot of space. However, there are ways for the defense to account for these weaknesses by changing up who has Force responsibilities, and that, in turn, switches up the run fits and coverage calls.
Cover 3 Buzz
The two main Cover 3 variations are known as Buzz and Cloud. Like with Sky, the first letter in the call dictates which player has Force responsibility (Buzz=Backer Force, Cloud=Corner Force, Sky=Safety Force). Each time Force responsibility is changed, the underneath coverage responsibilities change as well. Remember, defenses need to make their run fits and coverages integrate - we can't put players into impossible binds with too much ground to cover.
Buzz is a good introductory Cover 3 variation, because the coverage shell isn't altered very much - the same pre-snap three-deep shell stays the same. Two underneath defenders who start off next to each other simply swap responsibilities. Easy enough, right?
Cover 3 Cloud
"There is a lot of flexibility for changing the force in this defensive scheme. If the offense picks up on the fact that the free safety is your weak side force man then they can develop schemes to make it hard for him. All we do then is to change the force on the play from the free safety to the corner in order to switch it up.
We can also change the force by slanting the defensive 3 technique and 5 techniques inside and scraping the WLB outside. We bring the corner off his wide out and make him the force man. He is playing the number 2 wide receiver out of the backfield and thus is the force man on the run to him. The free safety then rolls over the top of the corner into the deep half of the field. If you don’t want to play him in man-to-man you can play zone. With this kind of force change you can play quarter zone coverage to the strong side and half coverage to the weak side." - Pete Carroll
Cover 3 Cloud is different in that it inverts one of the deep third zones. The cornerback, who takes Force responsibility, also becomes responsible for the Flat. Instead of coming down into the box, the safety will roll over the top of the Force cornerback and takes deep third responsibility.
Cloud is a great answer to formations featuring 2x1 receiver sets with a slot receiver (like the above picture) . A traditional Sky or Buzz call would leave the underneath safety and linebacker to the multiple receiver side in a bind, forcing them to either split out and distort the defensive front (allowing coverage against the two wide receivers and leaving bigger gaps against the run) or to keep the front intact and pretty much cede all short passing to the Curl/Flat.
Making a Cloud call and giving the cornerback Flat and Force responsibilities eliminates the to displace the other box defenders and the run fits become cleaner. The corner/safety coverage inversion essentially creates a Cover 2 type of exchange to that side of the field (although it should be noted that it's dangerous to run Cloud to the wide side, as that's asking the safety to cover a lot of ground).
Seattle and the Cover 3
It's been no secret that Gus Bradley is installing the defense that he ran as defensive coordinator in Seattle over the last four years. Like I've written about before, Seattle did some unique things with their defensive front, employing two-gapping defensive linemen out of a 4-3 base.
Here, Seattle is running a weakside Cover 3, with the free safety coming down into the box as a run support player (making him the Force player on weakside runs and the cutback player on strongside runs). Any runs to the strongside make the strongside linebacker the Force man. Lastly, with the strongside defensive end two-gapping (which turns into a sort of inside slant post snap against reach blocks that draws a double team) allows the middle linebacker to scrape cleanly as the Fill defender.
By playing with an eighth man in the box and two-gapping with the defensive front, Seattle is essentially matching the offenses numbers on both the strong and weakside of the offensive formation. Either a strongside or weakside run should funnel to an unblocked defender.
Football strategy remains cyclical, with offenses and defenses engaged in an everlasting battle to one-up each other. For the defense, much of this strategy relies around making one simple choice: to defend the middle of the field with inside/out leverage, or to play outside/in with top/bottom leverage across the field.
In the next piece, we'll be looking at the strengths and weaknesses of Cover 3 as a pass defense, how to cover up those weaknesses with pattern reading, and why/how Seattle runs press coverage from Cover 3.