X-sync for flash bulbs only when the shutter is at 1/30th sec. or slower. X-sync all the time for electronic flash like your Metz unit. If you use M-sync for electronic flash, the flash will have fired before the shutter opens and you'll get nothing. The delay built in to M-sync allows the relatively slow burning flash bulb to start burning shortly before the shutter opens so that it reaches peak output as the shutter opens. At slow shutter speeds, there is enough shutter open time for the flash bulb to reach peak output, with no need for the delay.