False Start or Illegal Motion

[kdf5]

Motion penalties are called against players who are already in motion at the time of the snap. What you describe sounds like a false start. Had the slot been in motion, clearly identified as a back, and turned upfield early then you have a motion penalty which is a foul at the snap and the play is allowed to continue because, like Patton said, A has a chance to correct the foul before it happens and you don't know when it's going to happen since you don't know when the snap will happen. A false start is something that simulates action at the snap. Moving a split second early is a false start.