X sync means simply that the flash contacts close and trip the strobe when the shutter reaches it's maximum opening. F sync. works a bit differently.
The flash bulb does not excite or reach it's point of maximum illumination instantly, it must begin receiving current 20 milli seconds before the shutter
reaches it's maximum opening to be able to deliver the greatest amount of light through the wide open shutter. There is no delay in X sync. when the
shutter reaches its widest opening and before it begins to close the strobe dumps the capacitor stored energy to the tube.
Have a good day.