Both the offense and defense crouch prior to beginning the play. This crouching action is a form of isometric press which preloads the muscles. This will enable the players to drive forward more powerfully when the snap is made and they engage the opposing team. 
This is a cropped version of a public domain photo. The original is listed on Wikimedia Commons as: <a href="//commons.wikimedia.org/wiki/File:Football_play_from_scrimmage.jpg" title="File:Football play from scrimmage.jpg">File:Football play from scrimmage.jpg</a>
<a href="//commons.wikimedia.org/wiki/User:RickyBennison" title="User:RickyBennison">RickyBennison</a>
This file has been <b>extracted</b> from another file