The "search in forums" switch at Admin Panel -> Settings -> Switches enables an easy way for visitors to search for topics in the forum they're viewing. An area appears at the bottom of the forum in which they can type their search term, select the forum to search in (defaults to current) and then perform their search.

This is just a simple search. To do advanced searches in a forum, they can go to the advanced search page and select desired forums on the right side of the search topics section.