Why the Nikon D5000’s maximum flash sync shutter speed is only 1/200th of a second – Thomas Comerford:
Because it has a mechanical shutter, and at shutter speeds above 1/200 the shutter doesn’t expose the entire sensor in one go. Instead, it forms a slit that moves across the sensor exposing each part of it in turn. So if you fired flash during a 1/400 exposure, it might show up in the top half of the frame, or the bottom half, or somewhere in between, but never the entire frame.