It checks the Permalink and redirects to the new URL, if it doesn't exist. It sends the header message "moved permanently 301"
When you change the permalink structure then this is a problem for bookmarks of users and also for search engines to link to your posts. This plugin uses the slug of the new url and search for a ID in the database of WordPress. If it finds a post according to the slug, the Plugin will redirect to the correct post and send a header message "moved permanently 301" to change the url on the index of search engines.