Re: Flash sync thingy
The reason is that with a focal plane shutter, which is the norm nowadays, the shutter speed is set by lengthening, or shortening, the time taken by the two blade shutters to pass across the focal plane. That is, shutter speed is determined by the "size of the gap" between the two shutter blades.
In normal flash usage the exposure is determined by the power and duration of the flash, which is usually much less than 1000th of a second, so that the shutter speed only needs to be set to have the shutter fully open when the flash fires. If the shutter speed is set too high one or other of the shutter blinds may still be over the focal plane when the flash fires, causing problems.
This still applies when using fill flash, and has to be compensated for by adjusting the aperture.
I hope that helps,