Back in the 1970s a “psychic” named Hoy (David Hoy?) was a regular guest on KMOX-AM the CBS owned and operated station in St. Louis.
He had started out as a stage magician, but apparently found it more lucrative to answer questions from the radio audience via his mystical powers. For instance, somebody would call in and say: “Where did I leave my car keys?” and he’d say “Look behind the sofa”. Then we’d never hear from the caller again, and everybody would act like this was something miraculous.
At least he didn’t tell people their keys would be in the last place they looked.
In any case, Hoy was something of an expert on old-time magicians, and he used to tell about a trick one of the greats–I believe it was either Houdini or Harry Blackstone, Sr., used to perform. On his first day in a town, he would hold a press event at a local bank. He’d write down a prediction on a slip of paper, put it in a box, and give it to the bank president to seal in a safety deposit box.
The next day the box would be opened with great ceremony and it was invariably found that he had make an accurate prediction about a surprise upset in a sporting tournament, the unexpected performance of a horse in a local race, etc.
What made it impressive is that the magician was not the one who lifted the lid of the box, nor was he the one who took out the slip and read it.
He was, however, the one who had palmed the original slip of paper as he appeared to put it in the box. And he was the one who rolled up a new slip of paper, with the correct prediction on it, and stuffed it into a hollow tube in the special key he used to unlock the box. When the key was turned the key shot the slip of paper into the box through the keyhole.
This may not be the exact method Copperfield uses, but you can rest assured he does something of the kind. It would be no great trick to get a notary to put his signature on two different slips of paper.
And yeah: that comment about 10% of the brain is a largely meaningless claim which has been refuted many, many times. I recall that the Society for the Scientific Investigation of Claims of the Paranormal (a group founded by Carl Sagan and Isaac Asimov, among others) published an article on this last year.