X sync would be for electronic flash, and fires the moment the shutter is triggered. M sync (magnesium) is for flashbulbs, prior to electronic flash, and the physical shutter opening is delayed 20ms from the act of tripping the shutter and the subsequent immediate triggering of the flash.
Here's an example of the sort of lens that would have both settings: http://www.flickr.com/photos/iantindale/463108245/in/photostream/
In other words, it is the shutter opening that needs delaying, not the flash firing. The flash has to fire slightly before you take the picture!