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Sometimes the offense will take the fullback off the field and replace him with another tight end. When they do this they are using what is called "max protect" - they have seven big guys to protect the quarterback instead of six. With two tight ends, the offense is said to be balanced. Also, the football team now has four fast receivers that have to be covered by the defense - two wide receivers and two tight ends.
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Sometimes the offense will also remove the running back from the field and substitute a third wide receiver. We now say the offense has an empty back field, because there's no one back there to help the quarterback. This offensive alignment takes a lot of the mystery away from the defense, as it's now obvious that the offense is going to pass the ball. However, since there are now five receivers on the field - three wide receivers and two tight ends - the defense is challenged to put at least five fast defenders on the field who can keep up with these guys. Some teams don't have five talented and fast defenders. This is a favorite formation of the Rams, who are a very fast team and like to challenge other teams to keep up with their track meet like plays.
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Sometimes the offense will remove the flanker from the field and substitute another tight end. The offense now has seven big guys on the line, and also a fullback and a running back. This is a lot of big strong guys for the defense to try to contain, and it makes it look a lot like the offense is going to run the ball instead of throwing it. The Packers like to use this formation, and put in a 340 pound tackle as their second tight end. They have 2200 pounds of men on the line, and another 500 pounds of fullback and running back coming up behind them. The Packers are very good at running plays out of this formation, and figure it's the defense's job to find a way to stop them.
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Football is like a big chess game - wizard's chess, I suppose. The offense is trying to get the football into the end zone and score points, and the defense is trying to keep them out. There are big and small wins and losses possible for each side. If the offense manages to move the football forwards four yards or more on a play, that's a small win for the offense. If the defense holds them to three yards or less, that's a small win for the defense. If the offense manages to move the football forwards 20 yards or more on a single play, that's a big win for them. If the defense manages to cause a fumble and recover it for their team, or intercept a pass so that their team has the ball, that's a big win for them. If the defense manages to get into the offensive backfield quickly and tackle the running back for a loss, that's a big win for them. Finally, if the defense manages to get into the offensive backfield quickly and tackle the quarterback for a loss, that's called a sack and it's a huge win for the defense.
There are a few philosophies on how to operate a defense. Two of the most popular are called "attacking" and "bend but don't break." In an attacking defense, the people on the defense will be constantly blocking and jamming receivers, and they will use many sneaky methods to try to get into the backfield and sack the quarterback. The favorite sneaky method is called a blitz, which is where you have one, two, or three linebackers, cornerbacks, or safeties abandon their normal post and rush in on the quarterback instead. This is a big gamble - if it pays off, the defense gets a big win. But a very good quarterback can often see this coming and throw the ball to a receiver who has been abandoned, which usually results in a play of 20 yards or more.
In the bend but don't break philosophy, the defense will pull back a bit and do everything they can to insure that the offense never moves the ball more than four yards or so on a given play. The idea here is that at four yards per play, if the offense has to move the ball 80 yards so score, they have to have 20 good plays pretty much in a row. It's believed that very few offenses can put together 20 good plays without making a mistake. The bend but don't break defense will be watching very carefully for these mistakes, such as a fumble or a poorly thrown ball that can be intercepted. It's important in this defense that the defensive guys themselves make very few mistakes.
In the picture above, there are four guys lined up in a straight line labeled E T T E. The letters stand for End, Tackle, Tackle, and End. These guys form the Defensive Line. They're very big guys. On football teams, we say there are fast guys and fat guys. These guys are some of the fat guys. We're going to start with them.