By default the plugin will fire the WordPress out-of-the-box hook for user_register. Some 3rd party themes and plugins that hook into the user_register action e.g. to send an email with a confirmation link, would run into the following error Fatal error: Call to a member function get_page_permastruct() on null when the action is triggered. This new configuration setting (on the Miscellaneous tab) – when checked – is a work-around to disable the action from being triggered (when a new user is created automatically by the plugin).
If you experience this error, you can try and tell the plugin not to fire the hook and see if the error goes away.
Simply check the option to tell the plugin to not fire the user_register hook.