Hahaha, I guess it sounded like that, but the boy didn't really move at all. I'm not exactly sure what removing the butt cheek did for the boy, but somehow he became more focused on his phrases in the music and less on his fingers.
At first I thought it was just this song, but I watched some of your other videos, and maybe its because you are under stress because you are being recorded, but you put an accent on the beats, instead of letting the phrase roll.
http://www.youtube.com/watch?v=imbwn6iVryQ
Do you hear how she lets the phrase roll into one another at the beginning, this is the sort of thing I am talking about. Its less of an accent and more of an impulse that you are putting on the notes. There isn't really much you can do about it actively, just be aware of it, and it will generally fade as you get better at your instrument. Or again, it could just be stress.
Another thing I was talking about was dynamic range. Your Forte should have grabbed my attention, and your pianissimo should have me almost straining to hear.
I play an instrument that struggles and works like hell to over come both of these things (tuba), so I guess they just stood out to me, since I've spent so much time getting past them.
Still though, for 14 years old, you are doing really nicely. Build that tech skill boy.
embarrass
But I personally think you should do larger dynamic contrast. 