The spectator "freely" selects "any" card from a "shuffled" deck and writes his or her "name" on it with a permanent ink pen. The "signed" card is then "lost" in the "deck."
But before all this, the (supposed) magician points out that on the table is a (pre-selected) card, folded into (approximate) quarters and horribly constrained within the (metaphorical) jaws of a (relatively) common household paperclip.
Once the (original) "card" is (allegedly) "lost" in the (Bicycle-brand) "deck," the "magician" (quickly) removes the (subtly curved) "clip" from the (currently) hidden "prediction" card, revealing it to be identical to the (unwilling) "spectator's" (previous) "selection" (apparently) including "the" (spectator's) "authentic" signiture.
Before performing the effect, the magician prepares two identical cards -- one is folded and inserted into a paperclip, and the other is left flat but attached to the fleshy part of the magician's right hand with another paper clip (note: this can be horribly painful).
After shuffling the cards, the magician cuts the deck and pops the flat card free from its paperclip with a flex of the hand, sending it fluttering to the table (the paperclip sails into the air, to be silently caught in the magician's pants pocket with a subtle turn of the hip). "I see one card has fallen from the deck," the magician says innocently. "Why not freely select that one so we can all save some time and remain good friends."
The spectator takes the "accidentally" fallen card and signs his or her name across it with a permanent marker. The magician then loses the card in the deck and, with great showmanship, removes the paperclip from the folded duplicate card. The folded card -- identical to the chosen card right down to the spectator's signiture (which the magician learned to duplicate beforehand) -- is revealed to the amazement of all.