Slow sync combines a slow shutter with flash. The flash fires and freezes your subject. The slow shutter records the ambient world around them in motion - lights trail, crowds blur, atmosphere builds.

Use rear curtain sync if your camera supports it - the flash fires at the end of the exposure rather than the beginning, which means motion trails follow the subject rather than precede them. More natural-looking.

Watch for: subject movement during the exposure - even after the flash fires, if they move, they'll ghost. Ask them to hold still, or shoot in a moment of natural stillness.