Is it offsides in American football to have one person start 20 yards behind the line of scrimmage and start running, having the ball snapped just before he crosses the line to give him a running head start? It seems like this would work since you are allowed to have one man in motion.
In the NFL, offensive players are not allowed to be moving towards the line of scrimmage when the ball is snapped. Quoting from the 2015 rulebook, Rule 7, Section 4:
ARTICLE 7. ILLEGAL MOTION. When the ball is snapped, one player who is lined up in the backfield may be in motion, provided that he is moving parallel to or away from the line of scrimmage. No player is permitted to be moving toward the line of scrimmage when the ball is snapped. All other players must be stationary in their positions.
Penalty: For a player illegally in motion at the snap: Loss of five yards.
According to this article:
Players in motion can do about whatever they want as long as it doesn't simulate what happens right after a snap. The catch though is that if the last thing he does is move towards the line of scrimmage, he must be set for at least one second. If he's moving parallel to or backwards from the line, he doesn't have to be set. Without this rule, you get Arena-style nonsense where a receiver is running at full speed towards the line right as the ball is snapped.
So, assuming you're talking about offense, technically it seems that you could run towards the line of scrimmage before the ball is snapped, but you would have to stop, set yourself for one second, right before the ball is snapped.
The rule according to Sporting News
Offenses often send players in motion prior to the snap because it allows the quarterback to see what type of scheme the defense is using based on how they react. Players also go in motion if the quarterback changes the play at the line of scrimmage. When players go in motion they must come to a complete stop and pause before the snap, otherwise illegal motion is called. This prevents wide receivers from sprinting towards the line of scrimmage at the snap to get a running start for the play.
A team A player is in motion towards his opponents goal at the snap
- or -
the player in motion is not 5 yards behind the line if he started in motion from a position not clearly behind the line and did not stop for one second to establish himself as a back