[AccessD] Access 2007 Forms as Tabs

Access 2007 provides the ability to open a sequence of forms such that each
form becomes a tab, such that with 3 forms open, say, across the top you
have 3 tabs, one for each form. (This is NOT the same as a tabbed form.) As
an example, the sample Time and Billing app does this.

I cannot figure out what property causes forms to open this way. I am
upgrading an Access 2003 app to 2007 and want to emulate this behaviour.

Can anyone suggest how it's done?

TIA,
Arthur