All Canon EOS cameras have two moving curtains in the shutter mechanism. The front curtain opens the shutter, and the rear curtain closes it. The normal operation of the shutter and flash causes the flash to fire immediately when the front curtain opens. This is Front Curtain Sync, and it's fine for most general flash applications. But say your subject is moving and you're panning the camera by using a slow shutter speed to pick up some ambient light. Flash photography in this mode produces motion trails out in front of the object you're tracking to make it look like it's moving backward. The trick is to get the flash to fire right before the shutter closes, thereby showing the motion trails behind the object, and this is exactly what Rear Curtain Sync does. You can set this feature either on the camera or on the Speedlite, but the Speedlite takes precedence over the camera's settings.
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