How can I make this effect?

Hey

This is probably one of those questions where you experts think: what an idiot! It’s so simple! Well it probably is, but I’m a noob @ this, so plz help me out here.

http://www.limpbizkit.com

So how can I achieve the effect that appears when you press on one of the buttons. I think it has something to do with masking. And something else, how can I make it so that the red stuff slides back to the button and then the other red stuff slides out of the button you’ve just presses in stead of just disappearing suddenly. Can anyone help me?