I have commented the code to a point but if there is anything you specifically dont understand just ask. Ive mad a simple masking transition and simple preloader(easy to swap with loading bar/animation etc).
The file i have uploaded will stop working if you try press any buttons above 4 as i have only made swf`s 0 ->4.
http://www.gifsrus.com/testfile/fadeinout/gridbuttons2.swf
in each swf first frame is blank with a stop() action, second frame contains the picture.
