When the light is low, the subject is moving and the flash just ruins the shot it is not the end of the world. Drag the shutter. The longer exposure will blur the image. Depending on what you want to achieve it can create a sense of, but not limited to movement, speed, chaos and time passing.. Spoken like a true master eh? LOL!
With the number of shots I had to make before I got this I can’t really call myself a master. However, it does really require a little bit of experimenting. A tad too long and the ghosting will be too much, a little to short and it will look like a panning shot gone bad. Just keep shooting. Throw the rules out the window, but make sure the viewer can still see a little definition of your subject. But then again I have already broken the rules so you can pretty much destroy it if it satisfies your vision.