Question about the way that wp_register_script works

Ok so I’m using the Roots Theme for WordPress (https://github.com/retlehs/roots), which, is sort of a starter theme or theme framework.

I’m confused about the way that it deregisters the WordPress loaded version of jQuery and but it registers it again without specifying the exact location of the script.

So in the header.php of the theme there’s this code:

...
<script src="//ajax.googleapis.com/ajax/libs/jquery/1.6.4/jquery.min.js"></script>
  <script>window.jQuery || document.write('<script src="<?php echo get_template_directory_uri(); ?>/js/libs/jquery-1.6.4.min.js"></script>')</script>
...

,which, reads to me: ‘download the Google hosted jQuery script but if for some reason the jQuery object isn’t present then just create a script element that loads the script that comes with the theme.

However, inside a theme file called roots-cleanup.php that is included in the functions.php of theme there is this code that deregisters the jQuery script that comes with WordPress but immediately registers jQuery again without really specifying the script’s url:

...
function roots_head_cleanup() {
  ...

  // don't load jQuery through WordPress since it's linked in header.php
  if (!is_admin()) {
    wp_deregister_script('l10n');
    wp_deregister_script('jquery');
    wp_register_script('jquery', '', '', '', true);
  }
}

add_action('init', 'roots_head_cleanup');
...

Now I went to see what the documentation says about this and it says something like: “This parameter [the location parameter] is only required when WordPress does not already know about this script.”

So I’m wondering, by including the script tags that load jQuery in the header.php does that mean WordPress is aware of them as far as calling wp_register_script goes? Like so:

wp_register_script('jquery', '', '', '', true);

I’m asking because I wanted to try this technique but I didn’t get the results I expected. Namely, javascripts that I included using wp_enqueue and that depended on jQuery still loaded even when I removed the script tags that includethem in the header, which, means the javascripts I loaded failed because there was no jQuery being loaded. This also tells me that WordPress isn’t necessarily aware of the jQuery script if I don’t specify the location paramater.