The idea is to show motion by having something moving in a specific direction, letting it blur in ambient light, and having the flash stop it at the end of the exposure.
That doesn't look too bad Kev, your shutter speed is 1/2 so you will always get a bit more blur, but I often do this too - I actually slow it down even more. Can get some cool effects and it is always fun to experiment.
I haven't been going that slow shutter ... instead more like 1/30, but I do like rear curtain sync for getting that ambient light in before freezing the subjects.