Subjects in Motion – Slow-sync Flash
Using flash during a long exposure can produce interesting effects, producing a sharp flash-lit image combined with a blurred ambient one.
Determine what constitutes a good daylight exposure first, and stop down by -1 to -1.5 stops to subdue the background, then adjust the flash (by setting the EV compensation to -1, for starters, or try different manual levels) until you have a pleasing balance between subject and background illumination.
If you can set your camera to rear curtain sync, then do so: the flash will fire at the end of the exposure rather than at the start, resulting in a more natural- looking blur which seems to follow, rather than precede, the subject. In this image, daylight is the junior partner and provides fill lighting for the flash.