Here are two approaches:
1-Shoot yourself playing the piano and duplicate the clip. rotoscope yourselef so the top layer is just you and the bottom is the whole clip. Then you can make the light streak go behind you.
2-Shoot yourself in front of a green screen and key out the green, then add the background. Then you can make the light streak go behind you.
Hope this helps and best of luck.
Stephen Smith
Utah Video Productions
Check out my Motion Training DVD
Check out my Vimeo page