Slow sync flash is the name given to a technique which combines long exposure times with the use of flash. The basic technique is to make a normal ambient light exposure and fire the flash at some point during the exposure period. The subject is correctly exposed in the normal way, but the foreground elements within the range of the flash are illuminated by the flash output.
The technique can be used to freeze fast moving foreground subjects since the pulse of light emitted by the flash is very short. Blurring caused by movement during the ambient light exposure can be controlled by careful choice of shutter speed and aperture settings. A long exposure period (slow shutter speed) implies a small aperture and the need for greater flash output. However, a small aperture records potentially longer but less-well-defined movement blur.
An important consideration in slow sync flash is at what stage during the exposure period to fire the flash. Many cameras offer two options - first curtain sync (sometimes known as leading curtain sync) and second curtain sync (sometimes known as rear-curtain sync. However, with exposure periods greater than one or two seconds it is also possible to fire a flash manually at the appropriate moment.