Slow Shutter Speed and Rear Curtain Sync
Rear curtain sync is a strobe synchronization setting. It is often used with slower shutter speeds from about a tenth of a second to a half second or slower.
Without rear curtain sync, the strobe is triggered when the shutter is first opened. Any motion blurs recorded as a result of using a slow shutter speed would appear to have happened after the image frozen by the strobe. With rear curtain sync selected, the strobe is triggered at the very end of the exposure, just before the shutter closes, creating a strobe-frozen image at the end of any motion blurs. This appears more lifelike, with a blur leading up to a clear image. If you try slow-shutter strobe techniques without rear curtain sync, some of the images may wind up looking similar to how an audio recording sounds when played backwards, while other images may come out fine.
When using these techniques, sometimes the background gets blurred, and sometimes the subject or parts of the subject get blurred. If you pan the camera to follow the subject, the combination of panning and the extremely short duration of the strobe light combine to make the subject look perfectly sharp, while the background streaks and blurs. Sometimes only one part of the subject is moving; this could result in a blurred hand with a strobe-lit hand at the end of the blur, and the rest of the subject relatively sharply focused. Experiment; there are many possibilities.