After marking a text passage a tiny popup with your search engines will be shown. There you can select the search engine that will be used on the text fragment.
The exact click behavior, as well as the search engines shown in the popup can be adjusted in the add-on options.
As a small by-product there is the possibility to let Selected Search automatically copy the selected text to the clipboard (much like "autocopy" (https://addons.mozilla.org/addon/383) does, i guess)
The latest versions also support Tree Style Tab (https://addons.mozilla.org/addon/tree-style-tab), where new background tabs will be created as child tabs.