Airing between ‘A Christmas Carol’ and ‘The Impossible Astronaut’ this doesn’t necessarily mean that it occurs between those two points. All we know that this is a point at which both Amy and Rory are travelling with the Doctor.
The episode begins with Amy asking the Doctor if they can talk. The Doctor prevents this by calling for Rory, leaving us to wonder what exactly she wanted to speak to him about. She could have been about to talk to him about her pregnancy, concerned what effect the TARDIS might have had on it or if this occurs after ‘The Impossible Astronaut’ she might have wanted to talk about seeing him die.
Learning that Rory is beneath the glass floor, interacting with the TARDIS console Amy asks if he is helping the Doctor fly it. Given that both the Doctor and Rory are making adjustments, rather than touching the controls it is more likely that they are carrying out repairs or maintenance.
It has already been establish that Rory has a fairly good understanding of scientific principles, if not any practical experience. It shows a great deal of trust for the Doctor (and the TARDIS) to allow Rory to take part in these procedures.
During this process the TARDIS enters conceptual space (causing the ship to hum). Conceptual space is traditionally where concepts (colour, taste, weight, etc) can be compared.
This could be a layer of reality where abstract concepts and ideas might exist. It could also be similar to the Land of Fiction, where unreal things can exist. Since the TARDIS is unable to make an emergency landing anywhere else but itself it could mean that conceptual space is empty (at least no physical safe place for the TARDIS to materialize).
When Rory drops a thermo-coupling the TARDIS makes an emergency landing, within itself. This is similar to ‘Logopolis’ when the Master envelopes the Doctor’s TARDIS trapping it in a recursive loop.
Presumably the TARDIS is enclosed entirely within its own pocket dimension. It can’t re-materialize in normal space because it can’t leave its own enclosure. The Doctor initially believes that nothing with be able to enter or leave ever again.
This situation could happen to any set of PCs that have their own TARDIS. There are many reasons why their ship might be forced to make an emergency landing and there could be dire circumstances in which it ends up landing inside itself.
If they do learn how to get out again this could be a good defensive manoeuvre, since not only would the TARDIS vanish from reality but nothing can get in. It is possible that there are other Time Lords who ended up in the same situation during the Time War and are just waiting for someone to get themselves out of the trap.
Their isolation is quickly ended when an Amy from the future arrives. This foreshadows the multiple Amy’s in ‘The Girl Who Waited’ not to mention future Amy’s first words echo her introduction in ‘The Big Bang’.
It is interesting that Amy is attracted to herself. Not only is she complimenting her own appearance the Doctor himself identifies that she is flirting. Yet she is offended when Rory considers the possibilities of having two Amy’s around.
‘The Girl Who Waited’ plays with this notion, with Rory being fought over by ‘present day’ Amy and ‘future’ Amy. It is also worth noting that the Doctor doesn’t raise the issue here that two Amy’s can’t co-exist (although he is keen to get ‘present’ Amy into the past).
Future Amy explains that the exterior of the TARDIS (which is currently in the console room) has slipped forward in time, so that if you step inside you’ll end up a few minutes in the past in the console room.
This drift could have interesting consequences in other scenarios. If the exterior drifts ahead those inside the TARDIS could use the scanners to witness themselves leave the box or observe events that are about to occur.
It could also mean that future incarnations or other creatures could enter the TARDIS exterior and arrive in the TARDIS interiors past. While the time difference is short here the same principle could mean beings from days, months or years in the future could enter the TARDIS. This could get confusing fast.
As with ‘Time Crash’ the distortion in time causes information to materialise from nowhere. Here future Amy explains the situation but only by remember what she herself said. This could be another example of time using such paradoxes to communicate.
This ties into themes of pre-destination, which will eventually come to a head in ‘The Angels Take Manhattan’ and end the relationship between the Doctor and the Ponds.
The Doctor is focused on making sure that Amy enters the TARDIS exterior at exactly the right point to stop the time line collapsing. Amy also checks with her future incarnation what her first line is. In contrast future Rory is able to convey a lot of information without memorising it, stating that it just happens.
This shows two different approaches to pre-destination. One in which time travellers artificially make things happen as they experienced them and one where the traveller allows it to happen naturally.
The Doctor manages to free them from the TARDIS with a controlled temporal implosion, resetting the ship. His only problem is that he doesn’t know which lever will control it and prevent them from dying.
He gets this information from a future Doctor who instructs him to pull the Wibbley lever. Since the Doctor pulls the lever and then dashes into the TARDIS exterior (to give himself the information) we can assume that if he’d pulled the wrong lever death would have been instantaneous (thus no future incarnation could have entered).
We might be witnessing an elimination of possible futures. In each version where the Doctor pulls the wrong lever he and the Ponds die but in their near past only the Doctor who choose correctly appears. Therefore it only appears as if he is creating a paradox by giving himself that information (when in truth it was a process of elimination).
From the perspective of those in the console room the TARDIS exterior dematerialises. The implosion is not noticeable at all (possibly used to fuel its escape). The fact that it was temporal implosion could be mean that the time differential was consumed.
Another option to escape a similar situation could be to brutally sacrifice one version of the TARDIS control room (and its crew). Since the problem is that the TARDIS has landed in itself, removing its surroundings could allow it move on.
In short, if either the present or future TARDIS were destroyed its counter-part could escape. PCs could have to make a tough decision about which version gets to live. True, with only a few minutes to separate them the difference is meaningless but that will be small comfort to those who are about to die.