HELP with rollovers!

http://www.biomed.com.gt/swf/preview/images/opia.swf

I’ve been trying to understand this rollover effect on buttons (link above) for DAYS and I don´t seem to get it right, can anyone tell me which kind of action is this??? is there any tutorial I can read??? I’m LOST;(

[url=“http://www.biomed.com.gt/swf/preview/images/opia.swf”]