If you're conducting a search on Yahoo! and use one of the filters on the left hand side to narrow your search, you may now see Sponsored Ads. The ads will appear on filtered results for selected sites who are also Yahoo! advertisers. Here's an example.
Let's say you're searching for "argyle sweater." You filter the results by JC Penney. The ad that JC Penney placed for the larger "argyle sweater" search will also appear in the filtered search that display only pages from the company's website.
This won't change the bidding for placement in the original, broader search. But the competition will be nixed, once the filter is in place.
How do you get your site listed as a filter? Yahoo! isn't revealing the secret sauce, but says it uses many factors including listings' quality, popularity and user response to determine which sites get the filter treatment.