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When the quarterback steps up to the center and prepares to start the play, he has a lot of work to do. His first job is to look at which defensive players are on the field and where they are, and try to deduce what the defense is going to try to do. He may see signs that the defense is lined up perfectly to stop his play. In this case, the quarterback will call out an "audible." He will shout out code words that tell the other players on his team that he is changing the play. Most often there is a special code word, chosen that morning, to tell the players that the play is changing. Perhaps the code word will be "blue," so to change the play the quarterback might shout out "Blue 23, Blue 23, Blue 23." If the quarterback is not changing the play, he still wants to confuse the defense, so he might shout out "Green 17, Green 17, Green 17." The other offensive players hear "green" and know to ignore this, but the defense doesn't know the "hot" color and has to wonder.
Frequently, the quarterback has what is called an option play. He has effectively called two plays at the same time, and as he steps up to the center he will read the defense and decide which play has a better chance of working. The most common form of this is what is called a "play action pass." In this play, the quarterback will take the snap of the ball and step back three steps, where he will meet the running back. He will then put the ball in front of the running back. If the quarterback has determined that the defense is aligned to stop a pass, he will give the ball to the running back who will run with it. If the quarterback has determined that the defense is aligned to stop a run, the quarterback will fake giving the ball to the running back, then as the running back continues on as if he is running with the ball, the quarterback will pass the ball to a receiver. No one on either team knows in advance which the quarterback will choose.
Another popular trick of the quarterback is called the bootleg. In this play the quarterback fakes a hand off to a running back, then sprints out in the opposite direction, looking to run or pass. If the entire offensive line moves in the same direction as the running back, leaving the quarterback unprotected and with a completely unbroken view of the field, this is called a "naked bootleg."
Another important trick of the quarterback is called a "screen pass." If the defense is rushing with great success and causing him a lot of problems, the quarterback will use several screen passes to slow down the defensive rush. In the screen pass, a couple of the offensive linemen will pull away from the line and run out to one side of the field. This will often be the center and a guard. The running back will also run to that same side of the field, perhaps ten feet behind the offensive linemen. Because the two linemen pulled, there is an unprotected path to the quarterback and there will almost certainly be a couple of drooling and slathering defensive linemen running unabated at the quarterback, visions of ambulances dancing in their heads. The quarterback's job is to backpedal as quickly as he can, drawing these two linemen and hopefully a linebacker or two in his direction. Just a fraction of a second before they can hit him, the quarterback will toss the ball to the waiting running back, who can now lumber up the field with 700 pounds of offensive linemen in front of him to protect him. This play, if executed well, will often go for a 15 to 30 yard gain, which is a big black eye for the defense. After a couple of these plays, when the defensive linemen break through the offensive line they will hesitate and look to their sides to see if they are being tricked. So the screen pass is a useful tool to slow down the pass rush.
Different quarterbacks have different skills. Dan Marino was a pocket passer. He had a very strong and accurate arm, and an incredibly quick release - when he decided to throw the ball, it was gone that instant. However, he was not at all fast, so no one ever thought Dan would try to take off and run with the ball. Marino required very large and stout linemen to protect him. Steve Young and John Elway were very good passers, although perhaps not quite as accurate and quick as Marino. However, Young and Elway were also fast and powerful runners, so the defense always had to be wary of them taking off with the ball and running for five or ten or more yards. With Young or Elway as quarterback, the team could afford to use smaller, faster and more athletic linemen who were better at opening holes for the running game. If their pass blocking was less than perfect, Young or Elway could most likely evade a defensive player on their own. Finally, Kordell Stewart was a phenomenal runner, although not nearly as accurate a passer. Kordell could call what was effectively a play action run - his offensive line and running back would be all set up to run one direction, perhaps to the left. If Kordell saw that the defense was stacked heavily on the left, he would fake the hand off, keep the ball, and run himself to the right, perhaps throwing the ball after a second if the defense reacted well. If the defense was more evenly distributed, he would hand off.
If the play is a pass, the quarterback will take the snap of the football, and drop back a few steps. He drops back so that the receivers have a bit of time to get out into their running routes, and so that he has a bit of time to watch the defense develop. The quarterback will generally drop back either three, five, or seven steps. A three step drop back means the quarterback is going to throw the ball almost immediately, before the defense has a chance to figure out what is going on. He will attempt to find a receiver who has run out only about three to seven yards, and get the ball to him. If the quarterback takes five steps backwards, he's giving his receivers time to get ten to fifteen yards down the field, but he's also giving the defense more time to read his intentions. A seven step drop usually means there will be a couple of receivers streaking at top speed towards the end zone, and the quarterback hopes to complete a pass for twenty or more yards, gashing the defense for a big play. Whatever drop the quarterback uses, it's the job of his offensive linemen to keep him safe for as long as he needs.
Quarterbacks have their own book of tricks to use on the defense.
Quarterbacks have to be at least about 6'1" just so they can see over all these very tall linemen in front of them. It helps if the quarterback is a relatively big guy, perhaps 230 pounds or so, so that he can absorb a hit if the pass protection breaks down. And it's good if he's quick enough to outrun a defender for at least a few steps, buying time to complete the play. The quarterback is the only player on your team who will touch the ball on every single play. Because of this quarterbacks are often the highest paid player on the team, making as much as $15M per year.