Below is how the homepage should look once the arrow in one of the top boxes has been clicked.
An overlay should darken the rest of the page, the active top box and should remain at its normal level of brightness.
An expanded section should slide down to reveal further content.
You can also see the homepage in action on this link, click one of the arrows in the top boxes to see the illustrated funcationality.
The CMS user should be able to edit the following on the homepage.