My guess is that they monitor usage and other metrics to aid them in the decision whether to add the feature to the Google service.

Google Experimental Search, that’s the name of the labs-like feature for Search, is currently offering three different features of which only one can be active at any time.

You need to be logged in to your Google account and visit the Google Experimental Search page afterwards to add one of the features to your account.

You can add a feature to your account with a click on the Join this experiment button next to the feature title.

Available for selection are the following three features:

+1 button: The +1 feature was revealed yesterday, and you can take a look at our initial coverage of the Google +1 button here. The feature will be enabled shortly on google.com, and later this year on other localized Google Search properties. Users who join the search experiment can use the +1 button right away.

+1 is basically a social recommendation service that will be used by Google as one of their search engine ranking signals. Your Google Contacts will furthermore see your recommendations if they make related or the same searches.

Why do not you +1 Ghacks if you enable that feature by following this link: http://www.google.com/search?q=ghacks&pws=0

Keyboard shortcuts: The second experimental feature adds six keyboard shortcuts to Google Search.

The j and k keys select the next or previous result, o or enter open the selected result, / puts the cursor in the search box and Esc removes the cursor from the search box.

You basically go up and down with the j and k keys and press enter or o to load a search result. You can append the parameter esrch=BetaShortcuts to search results to enable keyboard shortcuts on that page, for instance:
http://www.google.comjjk/search?q=rattlesnake&esrch=BetaShortcuts

Accessible View: The third and final experimental feature combines keyboard shortcuts with improved accessibility of search results. It offers a similar set of keys to navigate the search results, plus additional shortcuts. This includes i or h to move between categories, = and- to magnify and shrink items, a to switch to Accessible search results and w to switch back to the standard results view.

This feature is only compatible with the Firefox web browser for now, according to Google’s experimental search page.

You can test drive the feature by visiting this link.

Have you tried any of the experimental search features? If so, what is your impression of the ones you tried?

Let me give you an example to clear things up. You may right-click any form field in Firefox and select the menu entry Add a Keyword for this search. Selecting this opens a new Add Bookmark menu which asks for a name and a keyword. I went to ebay.com and did a right-click on their site search. I entered the keyword “e” and the name eBay.

Typing in “e computer” would have the effect that a search on eBay would be performed with the search string computer. I only have to type this phrase into the address bar to perform the search. The same method works for other websites including, Google, Yahoo, IMDB, Deviantart, Myspace and Slashdot.

There is another possibility to use keywords. Every bookmark has a keyword object that is normally empty. By assigning a keyword to a bookmark you can use that keyword to open the url. If you use gH for Ghacks for instance it would open my site if you would type in gH into the address bar. Pretty nice for sites that you do visit a lot.

Last but not least you can add wildcards to urls as well. If you edit a bookmark you see that it asks for a location and a keyword. The locations is normally the url of the website and the keyword the shortcut key. A wildcard would be added to the location to further customize a search or visit a special part of a website.

The wildcard symbol is “%s” which means that a valid location would be for instance http://%s.google.com/. If g would be assigned as the keyword you would be able to access Google websites such as adsense (adsense.google.com) or video (video.google.com).

inspiration found @ Tutorial Ninjas