This chrome extension is intented to help you with the content you want to add to your evernote to not having it duplicated.
It has to work modes. Default is, check on demand, that means, the extensión will query your evernote only when you click on the extension's icon. But you can configure the extension to check every url. Obiously, this mode has a larger network overhead.
To change the configuration, just right click on the extension's icon and go to configuration and click on the checkbox that appears there. That's all.