This extension creates an on-the-fly Google Custom Search Engine, based on the links from the current page. Try visiting http://slashdot.org, then click the icon in your status bar (a "G" with a chain-link underneath). Enter a query in the textbox, and hit the Search button. You will see search results that are based on the news stories and other linked contents of the slashdot front page.
Alternately, you can check the "SiteSearch" box and search over just the site you are visiting.
Accelerator keys are accessed using the Alt-key. For example, when the toolbar is open, use Alt-i to switch to site search. Note that "i" in "SiteSearch" is underlined in the user interface.
Current options on the toolbar:
* SiteSearch: If checked, a simple site search will be performed with the provided query, using the regular Google site: operator.
* URL Extraction
Exact: use extracted URLs without modification, to create a very "narrow" search engine.
Paths: use the host+path from an extracted url, but omit any filenames.
Hosts: use only the host from extracted urls to create a "broad" search engine.
* Extraction modifiers
Filter: some of the extracted URLs might be dominate search results for almost any query. For example, if we extract http://nih.gov, it is unlikely any other extracted URL will appear in search results. By enabling this option, the search engine will avoid certain overly-dominant URL patterns, like nih.gov/*.
Boost exact: this allows combination of host or path URL extraction, with an extra layer of exact URL extraction. Exact urls will be prefered in the search results.
* Other options
New Tab: open search results in a new tab.
For more info, you might want to read this blog post: