Incognito mode is great when you want to browse with a clean slate. However, this also means that all websites you visit will have forgotten your preferences.
This extension simply resets YouTube to widescreen (or "Theatre" or "Cinema") mode, and unticks autoplay. It also unticks 'Autoplay next video' on Vimeo.
NOTE: in order to actually work in Incognito mode, remember to tick "allow in incognito" in the extension tab.