Say you're with a couple of guildies and you want to farm bear mounts from ZA. You queue up for a random but get ZG instead. Bummer 15 min random dungeon cooldown right? Well just like direbrew you can queue up for the Headless Horseman specific dungeon. After killing the Horseman in just a couple minutes it will count as you completing the dungeon thus resetting your random dungeon queue. Now you can queue for a random ZA/ZG again, or any other random for that matter.
I know this was well known for Direbrew but I figured I'd post a reminder because people searching for Headless Horseman exploits might not find it.