Rear-curtain Sync
A setting that fires the flash at the end of the exposure rather than the beginning, so motion trails follow the subject rather than preceding it.
In depth
With the default front-curtain timing, the flash freezes the subject first and the ambient blur then records ahead of it, producing trails that appear to run backwards. Firing at the end places the sharp flash image at the leading edge of the movement, which reads as natural. The difference only matters at shutter speeds long enough for ambient light to register.
Examples in practice
A cyclist at 1/15 s with rear-curtain sync shows light streaks trailing behind a sharply frozen rider.