Second curtain sync is a camera flash synchronisation mode found on more advanced SLR and DSLR cameras. In this mode, the flash fires just before the second shutter curtain closes, rather than at the beginning of the exposure.
This technique is especially useful when photographing moving subjects with slow shutter speeds. It produces a natural motion blur that trails behind the subject, creating a realistic sense of movement. In contrast, using standard front curtain sync can make the motion blur appear in front of the subject, giving the illusion that it is moving backwards. Second curtain sync is widely used in creative photography, sports, and low-light action shots to enhance the dynamic impact of motion in an image.