Basically Google will be showing an alternate search box within the SERP (Search Engine Result Page). What this box does is to search into the particular site for further result. However not all site gets secondary search box to appear. At state, only those big shot will come with the search-within-search feature. Perhaps Google is running this on stages to test and see if it really works?
It might be a great feature that aid users in searching for a particular products but from the other point of view, is a big turn off for retailers and web publishers. User who uses the secondary search box, will eventually get result on search to a particular site (similar to site:www.microsoft.com document) only.
Here are some site (that I known of) that comes with secondary search box
- New York Times
- Associated Press
- Asus International