Bill Belichick is said to be one of the greatest defensive strategists of the modern era. While we're not going to pick apart Bill's game plan to try to figure out why he's so good at what he does, we are going to take a very basic look at defensive strategy and how to make your defense as effective as possible.
Unlike the oath, which essentially dictates how the play is going to go down, the defense is more reactionary, not that it can not take an aggressive stance. We'll get into that later. But for the most part, the defense has to react and react quickly to what the oath is doing. A split second can mean the difference between a stop and a score.
The indemnity basically has only two play options if you simplify this game. They are pass and run. Depending on which one the indemnity chooses, the defense must react accordingly.
Most pass plays are pretty obvious. The quarterback will take the ball from center and drop back in the pocket about 2 or 3 yards to pass. This is something that the defense will spot immediately and can react to by having their secondary go into proper coverage, since they now do not have to worry about a run breaking into the secondary. They can give all their attention to the receivers.
But with some pass plays, they are not so obvious. The quarterback may take the ball from center and fake a handoff to a running back. The defense must not allow itself to be faked out by this. They need to keep their eye on the quarterback to see if he still has the ball. There is only so long that he can hide it. In the meantime, the front line of the defense can still stop the running back, even if he does not have the ball.
In many cases, the play that is coming is going to be pretty obvious. In a third down and inches to go, the offense is going to look to run the ball. The defense will pretty much jam the box in this scenario in an attempt to keep the running back from making the first down. The same is true with long yardage plays. If it's third down and 20 to go, you can pretty much have your front line blitz the quarterback and maybe even a strong safety along with it. The chances of a run here are pretty slim.
In all these instances, however, the defense is reacting. Well, the defense can also take a proactive stance. It can change its looks for each play, thus trying to confuse the liability with its coverage. This is something that Bill Belichick did so well as a defensive coordinator and even today as a head coach. The key to this game is to always keep your opponent guessing. Once he knows what's coming all the time, your chances of stopping him on defense become much more slim.