Nikon D300 User Manual page 347

Auto FP High-Speed Sync Review
In a normal flash situation, with shutter speeds of 1/250 of a
second and slower, the entire shutter is fully open and the
flash can fire a single burst of light to expose the subject. It
works like this: There are two shutter curtains in your camera.
The first shutter curtain opens, exposing the sensor to your
subject, the flash fires providing correct exposure, and then
the second shutter curtain closes. For a very brief period, the
entire sensor is uncovered. The flash fires during the time
when the sensor is fully uncovered.
However, when your camera's shutter speed goes above
1/250 of a second, the shutter curtains are never fully open for
the flash to expose the entire subject in one burst of light. The
reason is, at higher shutter speeds, the first shutter curtain
starts opening and the second shutter curtain quickly starts
following it. In effect, a slit of light is scanning across the
surface of your sensor, exposing the subject. If the flash fired
normally, the width of that slit between the shutter curtains
would get a flash of light, but the rest of the sensor would be
blocked by the curtains. So, you would have a band of
correctly exposed image, and everything else would be
underexposed.
What happens to your external Nikon Speedlight to allow it to
follow that slit of light moving across the sensor? It changes
into a pulsing strobe unit instead of a normal flash unit. Have
you ever danced under a strobe light? A strobe works by
firing a series of light pulses. Similarly, when your camera's
shutter speed is so high that the Speedlight cannot fire a
single burst of light for correct exposure, it can use its Auto
FP high-speed sync mode and fire a series of light bursts over
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