I wasn't sure where it got the PR (though I assume it's from one of the many 3rd-party services, since the PR is fairly well known). So I went digging. I found this in the extension source code:

  this.getQueryUrl=function(){
    var b = "http://toolbarqueries.google.com/search?client=navclient-auto&hl=en&ch="+getGoogleChecksum(this.domain)+"&ie=UTF-8&oe=UTF-8&features=Rank&q=info:http://"+this.domain;
  }

Also, this is what the extension says about privacy [1]:

This extensions's privacy policy is no different than WOT or any other service or extension that is designed to bring you information on the sites you are visiting. Example, when you search on Google, it will always track your IP. It doesn't access your private data. Chrome shows a warning for all extensions that access browser tabs.

[1] https://chrome.google.com/extensions/detail/dbpojpfdiliekbbi...