The advanced search dialog has fields that will allow a more refined search, looking for documents with all given words, a given exact phrase, none of the given words, or a given file name (with wildcard expansion). All relevant fields will be combined by an implicit AND clause.
It will let you search for documents of specific mime types (ie: only text/plain, or text/html or application/pdf etc...)
It will let you restrict the search results to a subtree of the indexed area.
Click on the Start Search button in the advanced search dialog to start the search. The button in the main window always performs a simple search.
Click on the Show query details link at the top of the result page to see the query expansion.