it is usually a signal that he about to have the ball snapped to him.
if you watch the nfl and see sometimes when they do that but nothing happens they'll repeat it more exaggeratedly, depicting that someone didn't notice that and they should've.
Edit by smorin1487: if a quarterback is in shotgun formation, normally the Center is looking between his legs to get a perfect idea of where to hike the ball. The QB must life his leg to indicate that a player is allowed to go in motion... in this case the Center will be allowed to lift his head up to look at the defense, and then he can hike the ball.
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This is done primarily by the quarterback of the visiting team. This is to signal to the wide receivers and others on the team that the ball is about to be snapped. This is done by the visiting QB due to the volume of the home crowd.
They do this as a sign for the center to hike the ball to them. The stadiums are very loud, and opposing team will try to give fake signal calls to the quarterback.