Many NFL teams will put multiple players in motion prior to the snap, but only one player is actually allowed to be in motion at the time of the snap. If more than one man is in motion during the snap, a 5-yard penalty is assessed.
A man in motion is different from a shift, because a shift must be completed and the players must be set prior to the snap. A man in motion does not have to stop moving prior to the snap, but there may only be one player moving when the snap is made. The NFL mandates that only players in the backfield or off the line of scrimmage may go in motion, and that motion may only be laterally or backwards, never forwards. Prior to the multiple man in motion rule being implemented, quarterbacks would rely on the shifting and moving of their teammates to get off a good play. The NFL instituted the rule to create a sense of balance at the moment the ball is snapped, while leaving only a slight advantage for the offense.