Automatically equips your Onyxia Scale Cloak when needed.
The cloak will be auto-equipped upon entering Nefarian's Lair (after leaving Chromaggus' room).
See Custom Options tab for tank checkbox. If Tank is selected your cloak will be equipped when targeting Firemaw, Ebonroc or Flamegor.
It's up to you to put on your regular cloak when you're done with Shadowflame!