The current (hidden) search engine that is hard coded is the Bing.com/search site. This will eventually change to allow selection of different search engine.
The "site selection" dropdown allows the selection of several different sites on which to focus the search argument.
The search argument can be tailored to your specific needs. As an example your search argument will end up looking something like "site:nih.gov cancer vitamin c prostate." You can select another site and try your argument again and get different results.
I'm providing this tool, because it eliminates a lot of the SPOAFStating Personal Opinion As Fact websites that are popping up, and are filled with ads, and are just there to make money off the ads.
On some some sites you may have to reply to the simple CAPTCHA to continue. It's their way to prevent BOTSBOTS is short for roBOTSF7-1 from doing massive searches.
If you would like to format your own search argument for a different site here is an example you can use: (it will require you to respond to the CAPTCHA.)