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Of course, if the shutter is open longer than the flash exposure, the foreground that you flashed on is still going to get exposed while the shutter's open, so if it's a moving subject, it'll still get blurry around the edges.
Therefore, the reason you'd want to lock the exposure to 1/200th is when you don't give a damn about getting your background exposed properly and just want to make sure your main subject is sharp and clear.
The reason for 1/200th specifically and not 1/4000th is that the way focal-plane shutters like those found in most SLRs works is that there's a maximum physical speed for the shutter and anything faster is achieved by not opening it all the way.
So at 1/200th, the shutter opens all the way and so the whole sensor gets exposed when the flash goes off, but at 1/400th the shutter only opens halfway and moves across the frame half-open like that, so each part of the sensor is getting hit with light for 1/400th of a second but there's no time during that where the entire sensor is exposed. If you have a camera like this and fire an external flash above the sync speed, you'll only get a bright stripe across the frame where the flash went off and the rest will be underexposed.
Clear?
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