Okay, I've got the answer.
It's not a title of any kind of new Expansion title, but its a name of a deck created using powerful Basic Pokemon, usually Legendary Pokemon. While most decks revolve around 1 or 2 types, this one revolves around many different ones, usually 6. The cards that are used can come from many different expansion titles, and can even use Pokemon Cards from previous Generations. It's apparently had very hard deck to play around with, as you need a wide variety of Energy Card to play around with. The 6 Corner name came from the Variety of types used.
>A new deck design is emerging that attempts to harness the strengths of the many powerful Legendary Pokémon cards that have been revealed in the Black & White series. It’s called “6 Corners,” and its goal is to work as a counter for other popular decks making the rounds. What makes this deck interesting, too, is that the key Pokémon are all Basic Pokémon, giving players more space in their decks to be creative with the Trainer cards they choose to include.
>Up to the Challenge
>Before taking a look at the 6 Corners deck, it’s important to note that this is not an easy deck to play. In many cases, if you don’t know in advance what kind of deck you’re playing against, it requires you to make snap judgments early in a match as to which Pokémon you’ll face. Also, unlike most decks, which concentrate your Pokémon selection to a certain Energy type, the 6 Corners deck includes Pokémon that depend on many different Energy types. Most of the Pokémon you’ll be using have high retreat costs, which can magnify bad decisions when you choose the wrong one as your Active Pokémon. That said, the 6 Corners deck is also very flexible, giving you the tools to compete against a wide variety of popular decks.
To show you what I mean, here is an example of a 6-Corner deck:
As you can see, all the Pokemon are Legendaries, and the other cards are for support, mainly.