There was once a website that clearly explained how the Canon system worked,
it is gone now. It has something to do with the pixel shift system and how it
could time the readings of the CCD differently to get a progressive image in
combination with pixel shift. This resulted in some resolution loss, but not as
much as post de-interlacing. It also tended to look better (at least in my
opinion).

All in all it is/was a complicated system. I'm afraid that's the best I can recall
at the moment. I may have the information copied somewhere, I will see if a
search of my harddisks could reveal it (don't count on it though!).