As the extension description says, the extension relies on the fact of opening the current page via the HTTPS protocol. So if the page URL starts with `https:` (note the `s`), the extension replaces `http:` with `https:` in those HTTP links on the current page that point to the same domain (host) the page is located on.

The extension does not block anything. If you open a site via HTTP, the extension does nothing. Even if the same site is also available via HTTPS, the extension cannot determine the existence of HTTPS version of the current HTTP page.