Getting theoretical :External/Internal Transformations

[Pro]

Ever wondered why is it that switching between the two causes the player perspective in some (most) first person shooters to shift up/down a notch? Well I have. Explanation please?

[Doomulation]

My, my, shall I explain it to you? It can be very confusing to beginners.
Mmkay, here goes.
If you know it, when using 3D-graphics, use must use "transformations". The transformations is to be needed to make sure that all the graphics are put on the right place. Transformations can be done in two ways:
Internal & External

External means that DirectX does the transformations, which may not be just as accurate. Internal, however, means that the emulator does the transformations, which oftenly is much more accurate.

That is how it works. There's a lot of things to do when transforming. Which can cause troubles sometimes, but sometimes not. DirectX's transformations are not very accurate, and thus, graphics may be messed up.

I won't go deeper than that. But that explains it all I think.