you can no longer open the back of cameras to explain this like you could on film cameras
the basics are that the shutter is a blind that moves across the sensor plane opening it to incoming light.
Below 1/250th there is a moment where the entire sensor is open/exposed at one moment so when the flash goes off the whole sensor is lit.
Above 1/250th only a section of the sensor is exposed as a slit travels across the sensor exposing it to light, modern cameras prevent photos from being taken in this mode. Older cameras you would just get an a partially exposed piece of film depending on the shutter speed that was chosen.
Between the lens shutters do not have this problem as they always fully open for every photo. Hence cameras with between the lens shutters have a higher flash sync capability to focal plane shutter.
Hope this helps
"Photography to the amateur is recreation, to the professional it is work, and hard work too, no matter how pleasurable it may be."
( Total NIKON bigot )