EDIT 2016-10-5: This has been patched out. If I had known for sure that it would be, I could have skipped writing this post.
Each week, if you completed a Mythic+ dungeon the previous week, you will get a chest in your order hall containing a piece of gear corresponding to the highest Mythic+ dungeon you beat as well as a Mythic+ keystone 2 levels lower than your previous week’s keystone.
If you don’t have a Mythic+ keystone, you can get a +2 keystone by completing any Mythic or Mythic+ dungeon.
A few sources have reported to me that, as of this week, you could combine these two options. If you waited to open your chest until you did a Mythic dungeon, getting the +2 keystone, you would get both that +2 keystone and the Mythic+ keystone 2 levels below your previous week’s keystone.
It’s not clear whether this is an intentional feature or a bug that will be patched out. If you want to try your luck at getting a second keystone, this might be something you want to look into more. If it’s a bug rather than an intended feature, you probably want to avoid using it just to be safe.