The theroy on that goes something like this:
All energy and matter is related, and time (in essence) is just another,be it very descrete, form of energy. In that thread it is possible to convert matter into time, were it would be able to flow in the general direction of the (time)stream. it could then be re-materialised at any point, appesaring in any time. To the traveling matter no time would have passed, but to the outside world eons could have gone by.
The going backwards through time is explained by the loop theory. That time is just like like a piece of string tied together at both ends, so to travel backwards through time, you simply go forwards until the loop repeats itself, however if you managed to change something you would destroy time, because the loop is fixed in length and static in its own way.