This is a box or case that apparently holds two decks of cards, one red back and one blue back. It will permit you to change the deck of red backed cards in a very easy and simple manner right under audience view.
You can change your examined and shuffled deck for a set-up deck, or a trick deck like a forcing deck, Svengali deck or any other special deck. Or you can perform some novel magic using just two regular decks.
This resembles a “book” in a wooden finish, with velvet lined compartments for the performer wanting a superior quality prop.
This is not a trick – just an invaluable secret accessory for your magic, where a trick deck is to be used, and you want to imply it is a regular deck which has been seen and handled by the audience.