Trying to make a building preloader

Hey i have seen these around and short of wasiting the websites time with my question directly to them I ask here…

http://www.fuelindustries.com/main.fuel This one is a great example. It has an image that is masked at frames but how the hell do you tell the preloader to go to these frames and stop while such and such is loaded. I really wish Kirupa.com woulld add this to the tutorials list…