You get four red-backed cards, custom printed with red Bicycle back design, in classic gag card motifs: the 14 of spades, the 15 of diamonds, the venerable three-and-a-half of spades, and a very fancy Tree of Hearts.
(Note: these cards are NOT printed by the US Playing Card Company. They are imported, and while they handle well and smoothly, they have a slightly different finish from cards produced by USPCC. Also these cards are a tiny bit longer than standard Poker-sized cards. When in a standard deck, they protrude about 1/16 of an inch. So best to use them separate from the deck, in a prediction envelope of card box. Or in the deck, they can always be easily located and cut to.)
We magicians think these are just silly gags, but if played well, the revelation of the 14 of spades, or the three-and-a-half of clubs, or even the Tree of Hearts, not only amuses the lay audience, but can profoundly knock their socks off, too!
The effect as I usually do it: Spectator picks a card ("freely" selecting the 7 of spades). You explain that your facedown prediction card cannot, of course, be the same card, but it will be exactly double his card. You give examples, "if you picked the 2 of Hearts, my prediction would be the 4 of Hearts, etc". After the byplay when he names the 14 of spades, you show the correct prediction! Another great use is for a blendo card effect: two cards selected (again "freely", the 10 of diamonds and the 5 of diamonds). Via some sleight of hand, a card box, or maybe some other switch, you change the two selected cards into one card that equals their combined values -- the 15 of diamonds!
Tons of possibilities... Get a few sets, because you may wear them out -- you'll be using it so much!