Rear curtain sync mode is one of the creative techniques at your disposal when you work with flash. Basically, the flash fires off towards the end of an exposure, or just before the rear/second curtain closes.
Thinking of upping your flash game with some creative night photography? Night fashion portraits is certainly one of the fun projects you can do with flash. If you need some ideas, you might want to check out this quick video showing how to achieve cool creative effects using rear curtain sync flash!