This signal is used by the Referee to indicate a member of the offense was illegally in motion at the snap - usually towards the line of scrimmage (he may only be going parallel or backwards when the ball is snapped).
It is also used to indicate an illegal shift by the offense, when both hands are used simultaneously - ie more than one player was still in motion or that the offense had not set for one second after a shift and before the snap.
PenaltyThe penalty for either foul is 5 yards from the previous spot.
Begin the motion with your arm(s) held at your chest and move it (them) forward and level until an angle of approximately 45 degrees is reached in an unhurried manner.
Use just one arm to indicate illegal motion or both arms for an illegal shift.
Finally point to the offending team by stretching an arm out from the shoulder to the horizontal.