The animated image is 'scrolled' behind the mask image. The direction and speed of scrolling is configurable parameter. Film type animation may be achieved by creating a single 'film-strip' type image and setting the scroll amount to the width of each 'frame'.
Another parameter defines the active rectangle in which animation takes place. In the above example, the active rectangle bounds the ONTONET letters. Look at how it works for more information and another cool example.
Check out what the city looks like through binoculars.