Nice to see the light lines remained straight. On the other side, try this same technique in reverse sometime using a person in the foreground and set flash to rear curtain sync for the tail end flash. Zoom in, set manual focus on subject, zoom out, take shot and zoom in on 3 if its a 4 second shot.
Nice to see people out there playing and experimenting.
Really Nice job!
This looks like you had fun doing this one. Outstanding shot.