The theory is a slow shutter speed is used to capture 'ambient' (available) light, in this case the orange from the street light, which is combined with the camera's flash triggering as the shutter closes. This 'freezes' the action (in this case the sexy Skyline), this is easily seen by the nice silver paint on the Skyline due to the flash being daylight colour corrected. If this explanation confuses you, do drop me a line and I'll talk you through it.
