The answer to your question in it's simplest form is as flash sync increases faster than 1/250 second, the flash output decreases until at 1/8000 second the flash is extremely weak. The reason for this is the focal plane shutter is not fully open at one instant at speeds greater than 1/250. At the higher speeds the FP curtain slit moves across the film (sensor) plane so the light output from the flash has to be stretched so it uniformly exposes the plane as the slit moves. At the lower shutter speeds the flash fires in a single burst that increases in speed as required. Flash output is uniform.