Google recently added a small but interesting feature in its search results. There appears a small text box below result itself to search the site alone, Not all sites but google does have the feature on for few popular sites like Nvidia, eBay.
when we detect a high probability that a user wants more refined search results within a specific site. Like the rest of our snippets, the sites that display the site search box are chosen algorithmically based on metrics that measure how useful the search box is to users. We hope that you will make use of the site search box in order to get the information you’re looking for as quickly and easily as possible.