Moving arrow on buttons?

Hey I’m back after quite a while and need some info.

If you check out this site:
http://www.squarewave.us/

You will notice on the right hand side under the work section that a little arrow moves into place when a button is clicked _down the left hand side of the button.

Does anyone know how this is done? an fla would be great. I use mx 6 not flash 2004.

Thank you.