Yahoo's new contextual search tool lets users search for more information about a site's content without leaving the site. This new search tool can be embedded in a site by webmasters. Yahoo also has a toolbar users can download to do this type of searching.
It is definitely a different twist on the contextual search idea. It seems like kind of mix between ezula's infamous toptext and google adwords, with a toolbar thrown in for good measure.
Right now it's mostly a gee-whiz feature, but could potentially become a rival for google's adsense if yahoo decides to share cpc revenue with publishers who implement the technology.