Exerpt from the article:

... If he is still not sure at that point what the defense is doing he will simulate the beginning of a the play to coax the defense into showing what their intentions are.  Manning is looking for the reaction by the defensive players to the snap of the ball they thought was about to occur.  This is usually in the form of a false step or lean that a player will display when they are intent on reacting to the snap and getting to their assignment as quickly as possible.  The more things the opposing defense is trying to do, the more likely they will give things away....

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