Tutorial error?

Hi, im new to this forum, but i really love the site… the problem that i have is that i want to learn how to do this curtain effect like the effect on this site.
When you go to the main menu, and click on a link a little white box will appear… then before the actuall thing loads there is this box that tweens itself corner by corner… I have seen that type of effect everywhere but i have never seen a tutorial for it.

Could someone please turn me to the right direction?

thanx, usagi