2. High-speed sync flash is used when you want to use a shutter speed that is faster than your camera’s native flash sync speed, or when you want to use a wider aperture setting that requires a higher shutter speed as is often the case with outdoor daylight shooting.he flash instantaneously lights up the subject (it does travel at the speed of light), so the flash doesn't last nearly as long as the shutter remains open even though this is very fast (1/60s, 1/125s 1/250s, etc.). There is a limit to the shutter speed, and this is the camera’s native sync.