Board Thread:Theories and Speculation/@comment-30168130-20161223235115/@comment-4912646-20161230081651

To me, the fact that we see Obito's chakra emerge from his ashes and enter Kakashi establishes that it was something he could only do because he had just died moments ago, perhaps because his chakra hadn't yet faded from his remains. Otherwise there would be nothing stopping him (or anybody with a suitable technique) from leaving the Pure World and possessing people whenever they felt like it.