That extension allows to adjust a type of the internal interface (tabs and the navigation panel) in the alpha version of the new Yandex.Browser.
Works only in the alpha version of the new Yandex.Browser.
If you want to download that browser – http://browser.yandex.com/future
P.S. Keybind from keyboard like Copy, Cut, Paste, Select All and others features don't work in extension page – it's Yandex bug, I cannot do anything with it, but you can use right mouse click, and choose this options.