Illegal motion or shift?

[mbyron]

Originally Posted by Robert Goodman

I think you have to substitute the word "is" for "goes" above. Consider a player who's set just barely in the backfield and goes in motion parallel to the LOS. Suppose that at the snap the closest OL to him has body parts closer to his team's end line than some of his own body parts. He may have been a back when he went in motion, but by getting near that player is no longer a back.

That's not what the rule says. Here's (most of) 7-2-7:

Except for the player “under
the snapper,” as outlined in Article 3, the player in motion shall be at least 5 yards
behind his line of scrimmage at the snap if he started from any position not clearly
behind the line and did not establish himself as a back by stopping for at least
one full second while no part of his body is breaking the vertical plane through
the waistline of his nearest teammate who is on the line of scrimmage.

Nothing in the rules supports the imaginary idea that a back can lose his status as a back by going in motion. And if he's moving toward the NZ at the snap, he's still a back but the motion is illegal.