Strange pitching motion: balk or no?

[jTheUmp]

FED rules, not that it really matters as far as I know:

Right-handed F1 sets up in a stretch position, with his right foot in contact with the pitchers plate and his left foot directly in front of his right.
F1 comes set and pauses.
Then F1 lifts his left foot, steps 'backward' with his left foot directly towards 1st base, plants his left foot, then steps towards home with his left foot and pitches towards the batter.

I'm probably not describing it very well... it kinda looks like how a pitcher will sometimes step backward toward 1st when pitching from the windup position... only he was in the set position.

Is this a balk or not? I can't see anything against this in 6-1-3 or 6-2-4, but I do have a hard time parsing 6-1-3.