Ruby Sunday gave The Doctor a look as he plotted around the TARDIS interior. She worked up her courage asked him, "Doctor…what exactly is a dead-lock-seal? You said you would explain later…is now a good time?"
The Doctor pointed at her with a look of joy, so very pleased to be reminded. He declared, "Ruby Sunday, prepare to be answered! I have just the thing!"
The Doctor strode across the raised dais that housed the TRADIS' central column. With a flick of a minute lever a projection appeared, a young, blonde woman who smiled mischievously. She was dressed in normal, earth clothes, a baggy jacket and a simple shirt and jeans. The woman looked just to the left of the Doctor and asked the thin air, "Is this right, Doctor? Look here?"
A man appeared, buzzcut with a leather jacket. Mid-forties, red shirt and the kindliest face you might ever see. He smiled broadly directly at where the Doctor, the non-projection Doctor stood. He pointed and said, "No, no Rose. Just here. That's where I'll stand. Hello!"
The man and the girl both waved, and the Doctor returned the gesture. He turned and addressed Ruby and said, "I have about two-dozen of these Rubes. Not all questions can be answered in the thick of the moment. But! If not in danger, why not do it properly? This is the first record that I'll show you. This! Is what a dead-lock-seal is!"
He reached over and pressed a button on the console and the projection that had stopped in place jumped back into motion. The man stepped out of frame and the woman. Rose now addressed the Doctor. In a stilted, sort of speaking unpracticed lines from a script she said, "So Doctor, what exactly is a dead-lock-seal?"
The Doctor nodded at the projection and said, "Well! A dead-lock-seal is a type of lock that my sonic screwdriver cannot open, no device within reason could open such a seal."
The projection looked at the screen, nodded and then asked, "What do you mean? It's a sort of super-lock?"
The Doctor smiled encouragingly, needlessly at the projection and said, "In functionality? Yes. In principle? No."
He cleared his throat and continued, "A dead seal is like…melting a lock or…cutting wires. It is a physical action that renders the lock or door, whatever, impossible to open. Even if you have the right key, a dead-lock-seal will keep you out. Because the key is no longer part of the situation."
Ruby went to ask a question, but the Doctor made a placating gesture and pressed the button on the console again to continue the recording. Rose said, "So it's like if we're trapped and you try to open the door, but it's dead-lock-sealed its not you saying you can't unlock the door. It's you saying the door is unlockable, it is no longer possible. Like the lock on the door is just as useful as the knob."
The Doctor nodded at the projection before it disappeared. The Doctor addressed Ruby and said, "When I say, 'This door is dead-lock-sealed' it's just shorthand. It's like saying, this door is sealed in such a way that I cannot possibly open it any time frame that is remotely useful."