Basically slowing down or speeding up the line by line reproduction of the image, in order, top to bottom, or vice versa,
So, say, if the background is still and the foreground object is rotating, then it will come to resemble a spiral as the top part is rendered faster than the bottom part.
( WiL Pantheist Bi Polar Polyamoric Pansexual, Fri 6 Jul 2012, 10:09, Reply)

Coo, clever!
Ta.
(pissflaps., Fri 6 Jul 2012, 10:29, Reply)

You're welcome, I've seen it done a darned sight poorer years ago in some exhibition thing, the screen split into about 12 sections top to bottom.
Presumably the short cars bit is the same thing but done sideways.
( WiL Pantheist Bi Polar Polyamoric Pansexual, Fri 6 Jul 2012, 11:14, Reply)