I have a complex theory (and it may not make any sense, but here goes...). First of all, I know that yin and yang represent binary opposites (e.g. day and night, black and white, good and evil, etc), and that they cannot exist without each other. Therefore, I had an idea that maybe they were loosely based on life and death (plus Reshiram and Zekrom both look like angels). Then there is the speculation of N being a time-traveller, and the fact that TP look like they came from the past. Now, I am not a religious person, but, as life and death are opposites, and in the world of the living, our lives advance as we move into the future, I suppose it would make sense if the world of the deceased worked in the opposite manner (i.e. their "lives" advance towards the past, i dunno :S). This random theory could then explain/prove 2 things: the reason there is a beginning and an end to our lives, and that the past and the future (TP and N reference here) are also binary opposites, therefore fitting in with the yin-yang theme of black and white. I'm probably a
long way off the truth, though :S. It is just a theory, after all!