Summer 7 on 7 leagues and rule changes in the game have led to American football undergoing a slow and steady process of Euro-style evolutions in which players are groomed and equipped with skills from a young age in order to maximize their athletic potential and use it to obtain fame, scholarship opportunities, or even a shot at becoming a professional. The result is passing attacks that can shred opponents in a hurry and challenge the underlying assumption of defensive tactics that prioritize stopping the run while limiting big passing plays.
When the New England Patriots are taking down the NFL's greatest defense in a decade in the Super Bowl by relentlessly and precisely throwing quick passes to the flats, where defenses are daring you to throw the ball, you know the game is changing.
So what's the adjustment? Defenses need more players on the field who are good in coverage or else they have to give players that aren't great in coverage some really difficult assignments. Some teams could opt to find versatile players for every position but the preferable method for most teams is to align their defense in such a way that they have three good coverage players handling the opponents' best receivers and thus freeing up their safeties and linebackers to be traditional football players who are good against the run.
So the result is an iron law of defensive football in the modern era:
You have to have three good coverage players on the field to survive against the better passing teams.
The two corners have to be good, especially if the opponent has more than one good outside receiver or can move their best receiver around to create match-up advantages, and whoever is covering the slot needs to be strong as well.
Finding so many good coverage players is a real challenge that often separates the richer programs from the poorer ones, or allows particular underdog programs to stand out if they understand the law and how to stay in compliance.
In the modern era teams can often get by while just having solid players along the DL but there's no escaping your doom if you don't have some good players in the defensive backfield. An opponent will get their good receivers and passing game fixed on your poor DB play, run the ball well enough to keep you from diverting resources, and shred you.
Try to blitz them and you can just exacerbate the issue by short-manning the coverage against quick game staples that QBs can execute in their sleep. Unless you have players that can hold up long enough to take away the quick throws and buy an extra second for the blitzers, yes the rule of three makes for a better blitzing team.
Most opponents don't stack their two best receivers on the outside, 2014 West Virginia excepted, but will often put their 2nd best or even best receiver in the slot where they can counter-balance the outside receiver and help a team execute a quick passing game to march down the field.
How defenses handle the slot often determines their identity as a team. Here are a few ways that teams have looked to lock down the slot and observe this law that we've seen in the college game:
Option 1: The coverage safety
When Oklahoma determined that their 3-4 defense would work best in 2014 if they played Eric Striker as a "space-backer" to the field side rather than utilizing a nickel they were then determining that their 3rd coverage player would have to be the strong or field side safety.
With Striker coming off the edge, that left strong safety Quentin Hayes to cover the slot in what almost amounts to man-to-man coverage in their base 3-4 defense.
You'll also see some Cover 4 teams employ a similar strategy to handling the slot:
In both instances, the strong safety is dealing with that slot receiver with limited or even no help underneath from the strong side linebacker. In cover 4 he'll get help if there's no run action and the backer can help wall off and guide the slot up to the strong safety who has to be able to hand that player in open spaces in the deep field.
In cover 3 the safety is dropping down and relying on help inside from the linebackers or deep safety but is operating in a good deal of open space underneath.
The advantage here is that by deferring coverage responsibility back to the safety, the team can play a third or fourth linebacker on the field and close to the action. For teams that have big, fast athletes like Darron Lee or Eric Striker they don't want to take off the field this is an appealing option.
Option 2: The nickel corner
What if you don't have a great "space-backer" to put in the field but you do have a couple of great run support safeties you'd like to protect from coverage?
In this event, teams will often play a nickel corner who is ideally a player with a lot of short area quickness, good vision to read keys, and enough physicality to offer something in forcing or blitzing the edge. When that 3rd coverage player is a nickel corner, teams can stack the box with safeties in tight proximity to the line of scrimmage to control tight ends or run games.
That can look several different ways, a cover 4 team like Notre Dame would play the nickel in soft coverage and pack their safeties in near the box:
In this instance the outside corner isn't going to have deep help from the safety so he needs to be pretty good while the nickel can make sure to take away the deep outside throws to the slot, close on anything short, and force the receiver inside where he has help.
Kansas State in the Randall Evans era would typically play their nickel as a space-backer against two removed receivers but respond to opposing trips formations that look to create conflicts and run the ball up the middle by playing the outermost receiver in man-to-man coverage and canceling him out while turning nickel corner Evans into a de-facto outside corner:
Both of these strategies essentially dare teams to beat them by throwing the ball outside and underneath, which is a difficult throw for a college QB with low rewards if the nickel and outside corner can close on the ball quickly and tackle well. You're asking the offense to execute plays where the ball travels in the air an extra beat without actually going downfield, generally that's still a safe bet in the college game.
Cover 3 teams will follow this strategy as well and also be able to stack their safeties closer to the box while trusting the nickel to hold up underneath:
Given the open space to either side of the slot receiver, the nickel corner has to be quite good at controlling the slot receivers' initial release, breaking on the ball, and keeping an eye on the backfield to break on throws or support the run. However, there are a good many players who aren't fast enough to play corner and handle a good receiver without deep help but who are plenty quick, smart, and physical enough to play this role.
This style is better for taking away an opposing teams' quick game, unless they can punish the defense outside, at the cost of getting an extra linebacker on the line of scrimmage.
The difference between the varieties of cover 3 and cover 4 start to become obscure once you establish whether the D is relying on the safety or nickel to be the third coverage player. People face trade-offs and the personnel decision to go with a nickel is often dictated by a team having an excellent run-support safety and a desire to simply control the run rather than trying to overwhelm it with pressure.
As we've seen, having that third coverage player on the field allows a defense to feature the blitz, a really good outside linebacker, or a really good run support safety, all of which were otherwise threatened with extinction by the modern spread passing game.
Option 3: The ace up the sleeve
There is another way that can potentially allow a team to break the law against most opponents and avoid the challenging recruiting task of building a roster with multiple coverage players. A cheat, but not one you see very often.
That is, what if a team was able to recruit just enough athletes to have one truly excellent coverage player every year who was smart and coached up well enough to simply lock down the opposing teams' best player wherever they went? This would require either a team that could make rapid checks and substitutions or else that had a cast of versatile DBs that could play multiple roles in the secondary.
However, if a team had a player that could cancel out the best opposing receiver regardless of his position on the field it could be devastating. Here's how that might look...
The defense would have to rely very heavily on one base coverage so that they could train their DBs to learn multiple positions and then select the coverage athlete to serve as the "Ace" to follow around the best or most dangerous receiver. We'll use cover 4 for these examples, which has enough variety to allow a team to use a few different versions of it to get their Ace in the best possible alignment.
If the best or most dangerous receiver was lined up as the field receiver the D could respond like this:
If the best or most dangerous receiver was on the boundary you could press him up with the Ace and completely take him out of the game like Michigan State does against outside receivers:
If the best or most dangerous receiver was in the slot you could play the Ace in the nickel:
The more things the Ace can do, the more types of receivers that the defense could eliminate from the game. He'd ideally be good both in off and press coverage and if the team had multiple alerts and checks they could often protect him from doing anything other than locking down his target.
Their have only been a handful of players in college that could pull something like this off but few teams attempt to train up their DBs to even attempt anything like it. This would work best in a base nickel D so that the defense could adjust to motion without great difficulty since there would always be four other DBs on the field who understood multiple positions and could rotate to allow the Ace to follow a target.
If the offense constantly used motion to get the 2nd best receiver matched up against the 2nd or 3rd best coverage player, the defense would have to be willing to live with that but it would really complicate the offense if they had to rely on that tactic to get a favorable match-up for their 2nd best receiving option. Few college teams have enough motion in the playbook to bring enough to free up their best receiver from a defense that practiced this every week.
The goal of the "rule of three": Attack the quarterback
Very few college QBs have to deal with their security blankets being taken away and being forced to routinely rely on their second or third progression. Teams will generally cross train their better wide receivers to run different routes from different positions on the field so that they can always target their best players to accomplish different aims.
When a team can match up with the offense's top three receivers with solid to good coverage players, it really complicates things and can send a collegiate QB to a dark place, mentally. Some teams will do this with tight pattern-matching, most all are trying to do it by recruiting and developing as many good coverage players as possible, and perhaps more will try to match cross-trained receiving studs with cross-trained secondaries and "Ace" DBs.
At the end of the day, defenses that want to survive in the modern game will have to get back on the offensive and attack the quarterback's ability to quickly deliver the ball to open targets by either observing the rule of three or finding another cheat.