Slow sync is a feature available on a wide range of digital cameras that enables you to fire the flash at lower shutter speeds. You might wonder why would you want this ability. It permits you to keep shooting in a variety of low-light situations, thereby opening up new possibilities. By using slow sync flash, you can get a properly illuminated subject/foreground, yet still have the low ambient light in the foreground and background register in your frame. Many digital cameras let you set slow sync flash manually, but even on point and shoot cameras you have the ability to shoot at slow sync (typically marked as "Party" or "Night" mode on the Automatic Program selector). You probably haven't experimented with these modes on your camera, but give it a try. Review the results and think about how you can best use this new tool in your artist's tool box.（quoted from http://www.exposureguide.com/slow-sync-flash.htm)
