Any movement or location of a particular player that gives a hint as to the play that is about to be run. The defense will examine the formation in which the offense lines up, as well as the pre-snap movement, in order to gain an advantage and move quickly to deny the offense the ability to advance the ball. Typically, the movement of the running backs and tight ends will give a clue as to the type of run chosen and the location on the line that is likely to be the focus of the play.
Defenses will watch the pre-snap movement closely to try and get an edge on the offense and be able to react quickly to the play following the snap. The linebackers will adjust their formation depending on where the offense shifts to prior to the start of the play. A good linebacker will recognize the key and either call a different defense or adjust the current defense to be stronger at the point of attack. Offenses will combat this by using misdirection plays that run away from the indicative player.