This plugin disables the access to specified parts of the Joomla installation, if a configured security key is not provided. Just append the security key as parameter to the URL (e. g. https://myjoomla.tld/?thisIsMySpecialSecurityKey or https://myjoomla.tld/?key1=value1&key2=value2&thisIsMySpecialSecurityKey) to be able to enter the secured website part.
When the configured website part (possible: site, administrator, both) is requested and no security key is provided, the user gets redirected to a configured address (internal or external).
The key must be provided once per session, so if the users closes the browser window or he logs out or the server (e. g. Joomla software) terminates the session, you must re-provide the key once.
Read more https://extensions.joomla.org/browse/new/extension/access-a-security/system-block-access
