In front-curtain mode the camera fires the flash at the beginning of the exposure and, if you are working with a faster shutter speed, the flash light appears instantaneously as you press the shutter on the camera. If you set up your camera with a slow shutter speed and front-curtain flash, as described above, the flash will fire at the start of the exposure and capture the motion of the long exposure with the slow shutter speed. This gives you an interesting image, but potentially an odd and unnatural looking one, where the motion appears in front of the frozen subject.