A partir da descrição:
This extensions deletes all the cookies created in a tab (which are not used by other tabs) when you close the tab. In this way your privacy is guaranteed.
For example, as long as you stay on gMail, you are logged in, but once you close that tab (and all the others which are or have been on a Google site) the tracking cookies of Google disappear. Same for Facebook, once you close all the tabs which opened Facebook, its cookies disappear, so other sites cannot track you around the web.