YouTube use Flash Player extension forces YouTube to prefer Flash player over the default HTML5 player. flash player usually performs better on low end devices.
Not that this extension only requests Flash player on youtube's web site to be as lightweight as possible. This means embedded players on other websites still can use the default HTML5 player. To watch them in Flash just click on the video to let it open in a new tab.
For the extension to function, you need to refresh YouTube tabs