title: What does `git checkout-index -u` or `--index` do?
body: <p>The <a href="https://git-scm.com/docs/git-checkout-index" rel="nofollow noreferrer"><code>git checkout-index</code></a> command copies files from the index to the working tree.</p>
  
  <p>The documentation includes a cryptic remark about its <code>-u</code> or <code>--index</code> command-line parameter:</p>
  
  <blockquote>
    <p><strong>-u</strong><br>
    <strong>--index</strong><br>
    update stat information for the checked out entries in the index file.</p>
  </blockquote>
  
  <p>What is "stat information?" What's the difference between updating it and not updating it?</p>
  
  <p>I find that in simple experiments, <code>git checkout-index</code> just writes files to the working tree, as you'd expect; I can't see any difference between using <code>-u</code>/<code>--index</code> and not using it.</p>