Today during our assembly was the big test for this extension. Like other schools those related videos can cause quite an unneeded distraction. The extension worked like a charm.
What exactly was stripped from the YouTube video. There was no ad at the beginning of the video, there were no overlay ads during the video, and the related videos at the end were gone.
All you need to do is make sure your Gmail is synced in Chrome and then you can add the extension.