by Stijn Hommes

I think the best way to do this would be to use a stacked deck to gain control over the spectator's called out selection, so using the borrowed deck was the part of the effect that gave me a lot of headaches, until I came across Mnemonica by Juan Tamariz. His stack can be achieved by applying 5 Faro shuffles to a deck that is in "new deck order".

CAUTION: American and European manufacturers use different new deck orders and others probably do too. Make sure you know which NDO leads to the stack order you'll learn.

Needed: A deck of cards, a table and a spectator. Required skills and sleights: Faro shuffle, deck memorization, double lift, turnover switch.

Effect:
The magician borrows a deck of cards, checks if it's complete and removes two same colored cards from the deck. The rest of the deck is pocketed. Then, he asks the spectator to name out loud any card except the two already tabled.

A card is shown to have appeared face down between the tabled pair and when it is flipped over it matches the spectator's thought of selection.

Method:
First of all, you should try to get the spectator to hand you cards that are in "new deck order". This removes the neccesity of doing a fake check to put all the cards in order before the effect. If you need to do such a check, tell them you "need to check if the deck is complete". Seeing as they will be able to choose any card later on, this will make sense when they think back. When you have obtained the deck (and have done any required checks), you Faro shuffle it 5 times
and flash the deck to the spectator to show it is fully shuffled when in fact you know the place of every card in the deck.

When the deck is prepared you explain you sometimes do an experiment to test the magical properties of a deck and that you'd like to try this out today. You nonchalantly turn over the top card and place it back on top of the deck while acquiring a pinky break below the second card. "For example, same-colored pairs of cards can have strong magical powers. Let me show you." You double lift the top two cards off the deck, careful not to show the bottom to the spectators. Then you fan open the deck face up to find its mate, table it too and square up the deck again.

With your free hand, you casually take the card with the hidden indifferent card behind it, and slide it over to other tabled card then slide the whole pack to the side of the table. Your sandwich is now loaded. You fan out the rest of the deck on the table face up and explain the spectator they can choose any of the 50 cards remaining in the deck. This justifies moving the sandwich packet to the side, it reinforces the idea there's still 50 cards remaining in the deck and
again it makes the spectator believe the deck is thouroughly shuffled.

You square up the cards and pocket them (if it helps you can cut the deck in two and place the two packets in two different pockets to enable faster recovery of the chosen card). Then you ask the spectator to freely select any of the the remaining cards and name it out loud. You can even allow them to change their mind. After the spectator made their selection, you recap what happened while you use the time to recover their chosen card from your pocket. You don't show it yet. Instead, you move the sandwich packet back to the middle of the table and with your empty hand you spread it, to show a face down card has appeared in the sandwich. Since the spectator will assume it's their card, the heat will be off your hands, so you bring out the chosen card face down. Don't draw attention to it. Just casually use it to tip over their supposed selection. Exchange the two cards in a turnover switch and watch for the reactions.

New deck order version:
Some people will find memorizing a full deck beyond their capabilities. If that's the case with you, you can do 5 Faro shuffles as normal, flash the deck to the spectator to show it's shuffled and do 3 more Faro shuffles to return the deck to its original new deck order. The spectator will assume you're shuffling the deck further when in fact you're 'unshuffling' it. This will make retrieving their selection a lot easier. However, this means the effect will be weaker. While you look for the mate card, you need to be careful not to show the face of the deck and you obviously can't show the entire deck face up on the table. Instead of moving the sandwich packet to the side, you should drop the cards onto the table in a slightly haphazard fashion and use the squaring action to simply move them together into a neat centered pile.

Make sure you give the borrowed deck a good few shuffles before returning it, to ensure the spectator won't discover your secret.

Presentation notes:
You can change your patter to discuss the magical property of the joker cards to include them in your effect. This will allow you to include both the jokers and the rule card (as the 'selection') in the effect without messing up the the stack order when you choose the sandwich pair. If you do take two of the regular cards, don't forget to account for the tabled cards when you count to retrieve the chosen card from the memorized stack.

If you know the spectator owns a deck similar to yours, you can use that to your advantage and switch their deck with your pre-stacked one. If you get the chance to perform this effect with your own deck, you can use pretty much any stack you like because you'll won't be setting up the stack under the spectator's nose under pressure.

Helpful links:
Simulation tool for stacked decks.
How to memorize large numbers of playing cards.