With first curtain the blur occurs after the flash, if the flash is set at second curtain, the blur occurs before the flash.
So if a moving car was photographed using flash at a slow shutter speed and the flash was set to fire at second curtain then car could appear sharp but with a blurred image of the car behind it.
The idea being to create a sense of movement and speed.
Bright background lights.
Slow shutter speed.
Expose for ambient light.
Second curtain flash.
Main subject to be centre of frame.
Zoom from tele to wide.
Crop.
http://drewphilip.blogspot.co.uk/2011/06/flash-photography-tips.html
http://drewphilip.blogspot.co.uk/2011/12/outdoor-flash-photography-tips.html
http://drewphilip.blogspot.co.uk/2012/02/creative-photography-tips.html
http://drewphilip.blogspot.co.uk/2012_11_01_archive.html