Rear curtain sync is a flash photography technique in which the flash fires just before the second (rear) curtain of a focal plane shutter closes. This timing allows the camera to expose the ambient light first, then freeze the subject with the flash at the end of the exposure.
When using slow shutter speeds, rear curtain sync can create a motion blur trail behind a moving subject while keeping the subject sharply illuminated at the end of the movement. This effect is often used in creative action, night, or low-light photography to convey a sense of motion while maintaining clear subject detail.