Use rear-curtain sync when you’re combining flash with a relatively long shutter speed and want motion blur to look natural.

With front-curtain sync, the flash fires at the start of the exposure, freezing the subject first, and any ambient-light motion blur records afterward. That can make light trails appear to extend in front of the subject, which often looks unnatural.

With rear-curtain sync, the flash fires at the end of the exposure. The ambient blur is recorded first, then the subject is frozen at the end position, so the blur trails lead up to the subject. This usually looks more natural for moving subjects.

Typical examples include cars at night, cyclists, dancers, or any scene where you want visible motion trails behind the moving subject rather than in front of it.