The fullest explanation is in the novelization of Episode III. It's the only part I bothered with in the book store and luckily it supplied what I wanted. A scene that was likely in the screenplay but cut out because the movie desperately needed to end.
After returning from the Steel Mill planet, Obi-Wan meets up with Yoda.
"Asshole, dead is he?"
"Good as. I didn't actually do the coup de grace bit but he wasn't going to get far on foot or stump, if catch my drift."
"OK, your word I'll take for it. We're going to have to in hiding go until these kids grow up, packed to bursting with midichloriens hopefully. Or new hopefully, I should say."
"Damn, if I spend that much time on Tatooine, I'll surely age at double the rate for my species. The shit I do for this Republic.."
"Oh, way the by, did I ever tell you qui-gon worked out a method for life after death? It might come handy some day."
"Oooh! Lay it on me, froggy!"