Well, you see, when a Time Lord loses hand, and his immortal unnatural fixed point in time associate finds the hand, keeps the hand, brings the hand on board the TARDIS, and then doesn't argue when you insist on keeping it, and then the TARDIS winds up in the Crucible, and you funneled off your excess regeneration energy into the hand, and your best friend, who is human, touches the hand, WHOOSH. Metacrisis. Time Lord with human bits. Human with Time Lord bits.