As far as the TV show goes, unless I'm very much mistaken, we really haven't the foggiest how regeneration happens. (That doesn't stop us from making educated guesses, of course, but they could be Jossed at any moment.)

Perhaps the most extraordinary thing ever said about regeneration is the Ninth Doctor's remark to Rose at the end of Parting of the Ways: "I might have two heads. Or no head!" Whatever one's theory, that's a tough line to work into it. :)