Word search is invoked from the search toolbar:
When hitting the Enter key or pressing the Find button, all messages whose body
contain both words "searched" and "words" will be retrieved and displayed
in a new results page.
Also those words will be highlighted on screen within the body of
messages, until the user clears the toolbar's input field.
That functionality is useful to quickly retrieve messages that contain very specific keywords. It is not advanced enough to be effectively used with every kind of input, contrary to general-purpose search engines.
On a large database, the search will be faster if each of the words appear in a small number of messages, and also faster if there are few words to search for. It can be real slow when searching for several common words. Also, the more results there are, the more time it will take before the whole resultset will be available to the user.
The search is case insensitive and will not find numbers and words containing only one or two characters, or more than 30. It is also possible to specify words that shouldn't be indexed (and will be ignored in word search), commonly called stopwords. They have to be put into the non_indexable_words database table.
Searching for entire sentences is not possible with word search, but can be done using the general Query selection criteria, as a substring search.