When looking at defensive football from an offensive approach, you must first understand all the things that the defense must do. As a defense you must first be able to align correctly to any number for formations so that you are not allowing obvious advantages to the offense.
Secondly, you must make sure that you are both sound within your run gap/fit integrity as well as pass coverage.
This seems simple enough, but have you ever actually considered the numbers?
With a field width of 53 yards and a probable passing depth of 40 yards, you are asking 11 players to protect 2120 square yards on every play. Which breaks down to 193 square yards per player.
But of course you are stacking the numbers against the coverage guys when you take into consideration that the D-line is only expected to work within a rather small area.
Another largely advantagous aspect for the offense is action vs reaction. As a defense, a large part of what you do within a game is reactionary.
The thing about reaction is that as humans we must be taught to overcome our instinctive reactions in order to be productive.
Humans who work in dangerous environments must overcome their intial instinctive reactions to get the heck out in order to get a job done.
Same thing in defensive football.
As a teaching method, I will gather my qb's and receivers and conduct a simple experiment.
I will gather them around and take a tennis ball. While talking I lob the ball towards one of my players. As he catches the tennis ball I explain that he did not have to think about reaching out and catching the ball... that this was an instinctive reaction. This is what is the norm in terms of human behavior.
I then ask for the ball back and spit on the ball. I then lob it back towards the same player. This time the player will usually jump back and avoid the ball allowing it to land on the ground.
I then explain that this player has been taught that human spit is nasty, especially when it belongs to someone else. Thus, he overcame his intial instinctive reaction and a new knowledge based reaction has taken place.
This is the same thing in football.
If we are running a curl / flat concept we are attempting to pick on the curl / flat defender. In general teachings, this player is taught to cover the curl and tackle the flats. But, several high school age defenders are not taught (drilled) properly and if the threat of the flat presents itself they will cover the flat route, thus leaving the curl route open.
Those who have been taught (drilled) will show evidence by staying back and making you throw the flat route and then coming up and making a tackle.
That is bascially all defensive football is... aligning properly to the treat and reacting to any number of variables.