The Page Visibility API is used by webpages to detect if they are visible. Some websites, among them Twitch, use this information to pause video playback in background tabs. This add-on disables the API for all pages. Pausing playback of a video or other processing in a background tab is useful to save energy. So only enable this add-on if you can afford it. There are other ways to detect if a webpage is visible, e.g. onblur/onfocus. This add-on does not disable any of those other ways. So do not assume that this add-on will enhance your privacy.