It does it by storing "custom searches", that add some fixed terms to the items you want to search for in google.
For example, defining your search as:
"site:myweb1.org OR site:myweb2.org". You will find entries only in *.myweb1.org and *.myweb2.org web sites.
You can start your search from another application. Select text and use the "share" option to send the selected text to this MySearches.
This is useful, for example, when you are browsing the web and find the title of a book you are interested in.
The application comes with some default searches, but you can change them or add new (duplicate and edit) by long-pressing on any of them.
Books (spanish and english)
Movies (spanish and english)
This app shows adds. If that bothers you, you can disable ads in the configuration screen.