Not too sure about the technique, but is it likely it works because a lot, if not most, soaps are glycerine based? Glycerine, & glycerine suppositries are a remedy for constipation.
My only concern with this method is that the nature of the anus & rectum is to 'draw' things into it, and I personally would be a little concerned of soap getting drawn to far in, especially with regards to infections and reactions. That said, I have no medical training, so I could be talking complete tosh. :wink: