I have a D300 and while setting up my camera for fireworks last week I was dialing in some slow shutter speeds and in the process I went to 30 seconds, then bulb and then x-250. I didn't even know that was there. What is x-250? It would seem to have something to do with the fact that 1/250s is the maximum shutter sync speed but I don't understand the setting and how it differs from setting 1/250 as shutter speed. Can someone explain this setting? I couldn't find a real explanation in the manual.