The key to understanding the schematic bits of football is to remember the soul of the game itself: human confrontation in the form of single-play matchups.
Though it's a pure team sport, each play in football is won and lost based primarily on the 1-on-1s. Each coach will try to outfox the other with their play-call, and the players are left to identify their resulting responsibility and then execute it more effectively than the player across from them.
On the defensive side of the ball, your defensive backs (DB) are comprised of two sub-groups of players: cornerbacks (CB) and safeties (S). The CBs are generally there to match up with primary receivers, while the Ss are there to support the CBs or the LBs based on the play-call and how the play unfolds -- which might involve covering a receiver, joining a blitz, or moving into the box to help stop the run.
The main difference between a CB and a S is that the CB has better hands and can flip their hips more effectively (meaning they have a superior ability to both closely tail a receiver as they run their route and to make a play on the ball once thrown). As a rule, Ss are judged on their ability to read-and-react to the play itself, and on how hard they hit. They're often smarter and stronger, but less athletic. (Which is why it's not uncommon for a premier CB to become a S in their later years, such as with Charles Woodson.)
In a standard formation, you have two CBs and two Ss, with the remaining seven players being some combination of defensive linemen (DLs) and linebackers (LBs) -- split either 3-4 or 4-3 between those two position groups as the team's preference dictates. When the coach calls for a nickel defense, that means one of the guys in the box is coming off the field and an extra CB is being thrown in (making five in total, hence "nickel").
This typically happens on "passing downs", where the down-and-distance make it unlikely that a run will be called -- most commonly on 3rd-and-long plays. The idea is to expand your defensive coverage ability. If the offense puts five receivers on the field (some mix of WRs and TEs), a standard defense requires a LB to cover one of them. That's usually going to be a speed and nimbleness mismatch, giving an advantage to the offense.
Remember that every coach is looking to find an edge on every play. The scheme they use isn't terrifically important in itself. They're rarely expecting to pull off a true surprise. The idea is to get the most favourable one-on-one matchups available based on the roster they have.
For example, a RB who can catch is often a problem. When they line up behind the QB, their responsibility falls to a LB (who would be expected to either tackle or contain him behind the line-of-scrimmage). But if he suddenly goes into motion once the defense is on the field (and can no longer swap players), the LB realizes that they are now going into coverage. If that LB happens to be more of a pass-rushing type (strong with quick initial movement) than a coverage type (light on feet with good hands), the offense now has a mismatch to exploit.
This is why players who can play multiple positions well are so valuable. Clay Matthews, for example, goes to the Pro Bowl every year because of how equal his pass-rushing, run-stopping, and coverage abilities are. He'll never have 20 sacks in a year, but he's able to almost never fall into a full mismatch, which narrows what opposing coaches can do with their playbooks.
In the same vein, a team's roster usually includes players who have specific strengths and weaknesses that make them ideal for contextual situations and unideal for others. A coach can staff a nickel defense with any combination of CBs on their roster (they might have as many as seven or eight available), which he'll do based on the specific personnel and play-call that he's expecting to match up against on that play.
So there is no single advantage to particular formation or call. It's all an elaborate chess-match with 22 moving pieces. And that's why it's beautiful.
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