Think of a narrow slit moving across the film. Lines parallel to the slit will not be distorted.
Lines perpendicular to the slit will exhibit distortion, the specifics of which depend on the direction of subject and shutter movement.
As a shutter slit moves across the film, different areas of the film capture different points in time,
i.e. the 1/500th sec at the top of the film is not the same time as the 1/500th sec at the bottom.