Sessions in web pages. How to log out automatically

When we sign into gmail in one tab and orkut in another(remember both can only be of the same account. Logging into one automatically logs into another). if we log out from gmail and then go to the tab i开发者_开发问答n which orkut is already open, after remaning in the page for a few seconds the page automatically logs out. How is this done? i assume this is through page refresh but would like to know of any better way since i dont want to transfer so much data again and again.

Google's Single Sign On server knows that your account has been logged out, so next time any Google service validates your credentials, it returns that you've logged out so the app can act accordingly.

And about how to do it without having to refresh the whole page, it's very likely a XHR (AJAX request) which actually makes the page log out. It's probably not an explicit 'check for credentials' request, but any regular request that those apps have programmed by default (check for email on GMail, check for new friends or whatever on Orkut) that triggers the credentials check.