The way I understand it is the problem is they would be moving forward (or diagonally forward so as not so blatant) at the snap. Now if they move forward at all they will be required to come to a full stop or it's a penalty. I think. Not sure how the "abrupt" part is to be interpreted.
Relevant language: “Any eligible backfield player who changes his stance does not have to come to a complete stop prior to the snap, as long as his actions are not abrupt (false start) or forward (illegal motion).”