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.