How does Google get a toolbar right below the tabs in IE?

I researched through all BHO related documentation, but I just can't figure how Google gets the translation toolbar right below the tabs in IE.

Any useful pointers how to achieve the same effect for my own toolbar?

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Browser helper objects are COM objects that implement IObjectWithSite. During initialisation IObjectWithSite::SetSite is called with a pointer to the Site interface from which the BHO can QI for IWebBrowser2 and from there hook into browser events and manipulate the DOM document. It would be straightforward for Google's BHO to do just that, looking at the url or its content to pose some kind of helper message within the content itself.