Hey guys. You know that key you get at the end of Black 2?

Afaik it activates the challenge mode, but I'm unsure of this.

So, I have a few questions.

What does it actually do?

If it does activate challenge mode, can you carry over all your items/pokemon?

And most importantly

How the shit do you use it?

I looked it up and I couldn't find any useful info. I can't find it in my inventory either.

So, what the fuck?

Pick totally unrelated.