The way it works is that WP knows when you ask for a page that doesn’t exist.
If you ask for a page on your site that used to be non-wp page then the plugin will search for a WP page that matches the permalink.
For instance if someone tries to go to /kill_all_robots.html which does not exist, then the plugin will search for posts with kill, all and robots in the title. If it finds one, the user will be automatically redirected to the page. If there isn’t one it will search for a post with the key words in the text and redirect there.
The plugin relies on the url (permalink) having keywords that somehow match something in your WP blog.
It works great as long as the old system used descriptive urls for pages and the WP installation is on the same path that the old system was on.
This is an old plugin, but it still has thousands of users that rely on it.
Keith