Slider thingy, got me stuck!

I am trying to figure out how this guy made the image slider thing on this website http://www.perfectpixel.se/indexflash.htm
I know how to make it move, but if i put a rectangle over it and setmask… it doesnt allow me to mouse over the mcs and them do anything.
Is it just a normal mask? I am confused… and tired… hope to here from someone soon… maybe get a link for a tut or something