[nickrego]

This is the closest thing I could find which addresses this;

OBR 8.01(a): The Windup Position... he may take one step backward, and one step forward with his free foot.

As long as he delivers the pitch to the plate, I think I would not balk it. There really is nothing that says during that step forward, his pivot foot can not lose contact with the rubber. And it is not even mentioned for the Set Position.

I was able to reproduce this action quite easily. Just push off the rubber with a little hop and you get it. The pitcher probably thinks he is getting another couple of MPH on his pitches not realizing that once you lose contact with the rubber, you lose the power of your legs and hips. It is actually a disadvantage for the pitcher because he loses his stabilizing point. My guess is this kid does not get the velocity he could and probably does not hit his spots.

But I don't see how he is gaining an advantage on the runners or deceiving them in any way ? I suggest he gets a new pitching coach also.