slit scan is kind of like panning with a time component. If you pan, you let in light for the instant of the shutter being open. With slit scan, it's more like a scanner where you are gathering light for a longer duration, but only recording a vertical slice of information. This becomes more apparent when you look at his carosel shot, which you could never do with panning because your time component would overlap. Another example is an image of two cars driving in opposite directions. One car is elongated because it is driving with the film, another car is compressed because it is driving against the film direction.