A coin was put on the bottom of the cup in advance, which should actually be an iron coin. It takes magnetic force to bounce off, but it's definitely not the kind of signature.

Here's the process. The magician asks the audience to sign a real coin, then hides it, usually stuffing it into the sleeve, and then performs it. Then props and magnetism bounce off the iron coins at the bottom of the cup. The most important thing in this place is to scare the audience by performing. Before the audience's head turned to think again and again, the magician had picked up the iron coin and instantly replaced it with a signature coin. Here, the audience will never touch first.