light tricks

It's dragging the shutter, aka. rear curtain sync.
Not necessarily rear curtain sync. You can front curtain sync while dragging the shutter too. But regardless which you sync with (opening or closing) dragging the shutter is the process of setting slow exposures to bring either more light or sense of movement along with the initial moment which is frozen by the flash.

Just setting your camera to manual or shutter priority automatically enables this kind of feature.

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