This was already available from the Advanced Search on Google, but this is the first time that it has been seen on the main search results. The change has not been rolled out across all Google sites, it is only partially available on Google US – which  is normal procedure when testing new features or changes to the user interface.

You can see sites that are part of this experiment by google. Simply enter [site:www.example.com in the  Google search bar. There shouldn’t be any spaces between the : and the www.  If you see the drop down menu on the right hand side as shown in the picture above then enjoy the trial, if not, you can still see this feature in action within the Advance Search filters.

Information founded by SEO Consultant S. Inarejos