Rear curtain sync
Tweede-gordijnsynchronisatie · Technique
The flash fires at the end of the exposure, so motion trails appear behind the subject rather than in front.
With normal (first curtain) sync, the flash fires at the start of the exposure. A moving car then gets light trails in front of it, looking unnatural. Rear curtain sync fires the flash at the last moment: the subject is frozen at the end of the motion trail, with the streaks behind it.
Tools that help with this: