If the ball is moving foward from qb to reciever it is a pass. Anything else would be a fumble or live ball just like on a screen when the qb throws the ball slighty backward and short in the dirt it would be a fumble. This play the pass was slightly foward making it an incomplete pass.
Like a pitch for a RB is a fumble yet a swing pass to him, if properly executed, is an in complete pass. Both are quick out side plays to the back yet one is a fumble and the other an in complete. All decided by the path of the ball. Like a shovel pass, under handed pitch going foward that is really a pass.
Simpler said all foward passing is done behind the line and the passes are counted as incompletes where backward passing or lateraling is allowed at any time but incompletes here are counted as fumbles.