This means that if a white image is subtracted from any image, it results in a black image. A black image subtracted from an image has no effect. White areas in a subtracting image produce complete black, and black areas in a subtracting image have no effect whatsoever. The key thought here is, you're doing math with pixels . You're doing some very simple mathematics.
Difference is commutative.
Areas of maximum and minimum difference result in white and black pixels, respectively. This is a side effect of anti-aliased text. Since the jaggy edges have been smoothed with gray edges, to make the text look better, those grey pixels become black as a result of the channel operation. It really makes them stand out like a sore thumb. This artifact comes in handy for certain visual effects, particularly the Cows on LSD one.