Slow flash, which basically means, using a slow shutter speed while using flash, can be a great way to get creative with lighting. It can help you mix flash lighting with background lights, but it can also let you experiment with the blurred movement that can be captured with a slow shutter. It will contrast this movement with sharp subjects because the flash will "freeze" any movement.
The slow flash settings can easily be accomplished by:
1) Aperture priority mode
2) Turn on the flash
3) If your camera is a Nikon brand, then change the flash mode to Slow Flash