An interesting effect when photographing dim areas with background lighting.. nightclubs for example: - Set your camera for flash 2nd curtain - Set your aperture (letsay f/5.6) and meter to determine shutter. The shutter setting will most likely be very slow for handheld. Thats ok... Increase the shutter by a stop or two to intentionally underexpose. - Set the flash to properly expose the subject at that aperture The flash will freeze and expose main subject. Motion and background blur will also be captured.