This add-on sends you to a different website (google by default) after a certain amount of time has passed (3 hours by default) and makes YouTube inaccessible for a certain amount of time afterwards (1 hour and 30 minutes by default). The timer does not tick down while YouTube is open in another tab or if no tab with YouTube is open. The website you get sent to and the durations of the watch session and the inaccessibility period can be changed by clicking on the browser extension icon and selecting manage extensions > youtube watch time limiter > options.
