Rear Sync describes a method of flash photography.  It is used mainly to show motion.  In a standard flash photograph, the flash fires just once at the beginning of the exposure, usually with shutter speeds of 250ms or less.  In a Rear Sync flash, the flash will fire at the beginning as well as the end of the exposure, this is a great method to capture motion in a dimly lit situation or to get a more exposed background.  Its a tricky method to get used to, but once you get it, its a great trick to add the bag.

Nick