Transition between buttons and external .swf

I’m trying to build a flash similar to what can be found at http://www.fairorunfair.com/

I figured all the little people in the office scene are just simple buttons with movie clips imported into the different button states, and when you click on each person it loads a new external .swf. But how do they do that transition thing, where the person slides over and it starts playing a new clip? Maybe someone here could help me (T____T)