There are many qualities that a quarterback must possess. However, the most obvious is the QB's ability to throw the football. Throwing the football requires a tremendous amount of coordination and teamwork for proper execution. The QB can make up for some deficiencies with proper reads. Whether it is the Pre-Snap Read, Reading on the Move, or Adjustments in routes, the QB's recognition, anticipation and reaction are based upon his knowledge of the offense as it relates to what he sees.

Pre-snap read The QB must make a "Pre-Snap Read" confirming the defensive secondary's alignment. The PSR provides the QB with help in making the proper throwing decision; i.e., allows the QB to establish his thought process prior to the snap. There will be many times when the QB can determine what the coverage is before the snap. About eighty percent (80%) of the time the coverage will be given away by someone's alignment in the secondary, typically the second defender inside. Even when the total coverage is not given away, through observation of particular alignments, you will be able to eliminate some coverages or narrow to a "Hard Focus" area. The QB must approach the LOS the same way every play and get his hands under the center. The PSR process includes a "Soft Gaze" left, middle and right. The purpose is to identify (1) the depth of the corners, (2) number of safeties, (3) weakside flat defender, and (4) the number of run defenders ("front"):
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http://smartfootball.com/passing/attacking-coverages-in-the-passing-game