I first heard about from some of the Euro guys running WE's in Finland, Holland, and a couple from Northern Russia. You really don't want to use it if the temps are over about 40-45 degrees F (sorry, don't know the C equiv for that off hand).
My very limited testing was in my KC-02 at about 40F. I got 450fps on a .2g bb (which is about the same readings at about 75F+ and propane for my rifle)