I'm still confused about how the transfer of immortality actually happens. So when a geass user becomes powerful enough they can take the geass giver's immortality? Or does the immortal have to give their power to them? I got the impression the Nun simply gave her powers to C.C. and then killed herself. But then with Charles they made it sound like he simply took it from V.V. Since C.C. stated Charles "took" V.V.'s code and then asked him why he did it.