Nikon D300 User Manual page 752

What Are Monitor Preflashes?
When you press the Shutter-release button with the pop-up
flash open, the camera's built-in Speedlight fires several
brief preflashes and then fires the main flash burst. These
preflashes fire whenever your camera is set to TTL mode,
even if your D300(S) is controlling multiple flash units
through CLS (discussed in an upcoming section). The
camera can determine a very accurate exposure by lighting
your subject with a preflash, adjusting the exposure, and
then firing the main flash burst.
Your D300(S) exposes the sensor to light for specific periods
of time. This is controlled by the camera's shutter speed. The
actual exposure is handled by two moving objects called
curtains. The D300(S) has two shutter curtains. One gets out
of the way of the sensor to start the exposure, and the other
replaces it to stop the exposure. The first one is called the
front curtain and the second one is known as the rear curtain.
In this context, the "front" and "rear" are not important as
indicators of position but as indicators of which moves first
and which moves second. The flash must fire when the first,
or front, curtain is fully open and before the second, or rear,
curtain starts closing. The time between the front curtain
opening and the rear curtain closing is the actual shutter
speed.
The whole sensor must be uncovered when the flash fires in
normal Flash modes
fast, the rear curtain will closely follow the front curtain and
partially block the sensor when the flash fires. That's why the
shutter speed is limited to a maximum of 1/250s on the
D300(S). Faster than that and the sensor is always partially
(non-Auto FP). If the shutter speed is too
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