How is this done?

Hello-
I’ve really been trying to do this navigation in flash, with no luck :frowning:
http://www.sonyericsson.com/spg.jsp?..te=ph1&zone=ph

Does anyone have any insight or explaination on how this is done? Is it a bunch of Masks that once rolled over expand to reveal more beneath? I can’t seem to grasp the concept…Thanks in advance…