Nikon D300 User Manual page 201

Flash mode Description
In shutter-priority auto or manual
exposure mode, flash fires just before
the shutter closes. Use to create effect of
a stream of light behind moving objects.
In programmed auto and aperture-priority auto, slow
Rear-curtain sync rear-curtain sync is used to capture both subject and
background. Use of tripod is recommended to
prevent blurring caused by camera shake.
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