What is the Junk Yard curtain?
Short answer: The curtain is a slim strip of your scraps that slides in when your pointer reaches the docked screen edge (top or right). Click a tile to copy it, then paste with ⌘V.
The curtain is a slim, translucent strip of your scraps that slides in when you move your pointer to the edge of the screen it's docked to — the top or the right. Click a tile to copy that scrap, then paste with ⌘V wherever you were working.
It doesn't take focus from the app you're in, so you can grab a scrap and paste without switching windows. It closes on its own shortly after your pointer leaves.