I have not hear of an x-orgams. However, my wife has frequent orgasms from stimulating her a-spot. The a-spot is the anterior-fornix, which is the location where the vagina and cervix connect. Anterior meaning the front portion of the vagina/cervix. There is also something called the pfe-zone which is the posterior connection between the vagina & cervix.
There is a fair amount of literature out there talking about the a-spot and pfe-zone - it's possible that stimulating this area results in the x-orgasm.