title: How to activate the Bootstrap modal-backdrop?
body: <p>I want to put up an overlay on my screen while some ajax stuff is running.  Building my own overlay div isn't a big deal, but, since I'm already using Bootstrap, I figure I might as well use its <code>.modal-backdrop</code> -- it should already be around, and there's some value to consistency and all that.  </p>
  
  <p>The problem is I can't find the right way to do it -- I've tried adding classes to <code>.modal-backdrop</code>, or calling <code>.show()</code> on it, but it's not working.  </p>
  
  <p>Note that I don't want to bring up a whole modal dialog, but just reveal and then hide the backdrop.  </p>
  
  <p>Any suggestions? </p>