Google's historyFrame

Does anyone know how the Google services historyFrame works? If you Firebug Google’s services that make extensive use of AJAX (for example, Google Calendar), you’ll eventually come across a hidden iframe called the historyFrame. This frame enables the user to use the back and forward buttons even though the site is primarily AJAX based. All it contains is this:

Trying to find parent._goHist() in the deliberate maze of Google’s page source is a bit more difficult. I’m wondering if anyone’s already done it, written anything similar, or reckons they can have a pretty good guess at what it contains.