See, part of it is that the hand was still twitching when the jar got broken in that second TW ep. So people are free to speculate whether Jack does anything kinky with the hand, or whether he's planning to use it to find the Doctor, or -- my personal favorite -- whether he'd like to find alien tech that does the same thing as in The Fifth Element and basically use that hand to clone an entire spare Ten. (I nearly wrote a fic along those lines during the first month TW was airing, but only got one scene down and didn't come back to it.)