Lol for anyone else, not you Rab, who doesn't want to google it, the basic principle is keeping the intake valve open for a period of time during the compression cycle to reduce the compression therefore reducing load on the engine making it more efficient.
It's quite easy to turn a engine by hand with the plugs out but difficult or impossible in some cases with the plugs in.
Easier to turn means less fuel needed to overcome the cylinder pressure.
Potato version: Use less fuel when cruising.