"And what are you now?" the Doctor pressed. "Unless you're Time Lord, I'm afraid our technology is still going to be a bit beyond you. Oh, you might be able to understand a good piece of it, but when it comes to a TARDIS, there's no substitute for being ... well, like me." The Doctor raised his tea to his lips and took a sip. It wasn't bad. He'd still prefer a cup of Jackie's, though he'd never admit as much to the woman herself, but at least it was something.
"Though you may be onto something with that pocket universe theory," he conceded. "Might explain why the TARDIS can still draw enough energy from the vortex to survive and make small jumps, but not enough to escape. A pocket universe, being smaller, could mean that she's close enough to her native universe to pull a bit of power in."