i NOT UNDERSTAND

On Nokias with colordisplay they have something called a powersaver
("strømsparer" in norwegian), not the same as a screensaver, but...

Instead of the usual screen with operator, signal, battery, clock and
date, the screen is black with a clock jumping around the screen :-)

But how does this save power compared to the regular display?
The background light is switched off anyway.

Anyone tested?  Is somthing more happening in the background?