Motion is a type of shift, and it’s the only type of shift that is legal to be occurring at the snap.
See Redding’s interpretation in Calhoun’s post. It’s the exact scenario we are describing without having to pin multiple case plays and rules together to come up with a ruling.
Motion is not a type of shift and that's the whole point here and a source of confusion for too many officials. As his ruling says, in (a) it's legal motion, in (b) it's (a shift and therefore) an illegal shift because the snap occurs before one second elapses after he takes a new set position.
2.39
A shift is the action of one or more offensive players who, after a huddle or after taking set positions, move to a new set position before the ensuing snap. 7.2.7
Only one A player may be in motion at the snap...Motion is: still occurring at the snap. A shift is: the act of assuming a new set position.
Also, notice that in the interpretation we are to "consider" it to have taken place "while in motion" even though the example is clear that he has stopped and the snap does not occur "while (he is) in motion." It doesn't flat out say that it's legal, but instead that we're going to "consider" it legal because the intent of the rule isn't being violated. This is a good interpretation because we're not going to nit pick a back or receiver who goes in motion and pauses before the snap, as long as he is basically in the same position he was in while he was in motion (ie, a crouched RB shuffling side to side in a crouched position, a WR who is upright). But once that motion player makes a drastic change in his stance (ie, going into a 3 or 4 point stance) then we're no longer going to "consider" him to be in motion and it is now a shift.
Also, as for your comment to Calhoun stating that the forward motion is the reason for it being a foul: I'm sorry for digging into the books again (although I think that's how we should learn and base our rulings), but the best example I can find is 7.2.7 COMMENT:
If the quarterback drops his hands under the snapper without stepping forward, it is a shift and not motion then it references the definition of a shift and rule 7.2.6. This clarification makes it pretty clear that the action of "moving forward" has nothing to do with the ruling and that simply moving from one set position to another is a shift.
I'm not trying to nitpick (hopefully it doesn't feel that way and we're just discussing interpretations) and I feel we're pretty much on the same page for the ruling with this example but I think your interpretation of "moving forward" as well as saying motion is a shift aren't completely accurate and could mislead others reading this.