Well first....Derren Brown never did a magic square that fooled magicians
. He only did a great presentation of the square.
People who plan on hiring you will keep your card not because there is a trick on the back but because you're a good performer and that is what they want at their event.
Magicians who like to do tricks with their business cards and things along those lines are not paying attention to a major rule of marketing..."enter into the conversation they are having with themselves." This rule means you have to think like they are thinking.
Clients are not saying "Wow, since he taught me how to do a trick I will keep his card longer. I've always wanted to learn a trick." What they are saying is "Wow this guy is good. Maybe we should have him at our banquet in a couple of months. I'm going to give Amy (the decision maker) his card."
If you give people something for free then it has to be of value to them. Teaching them a trick has little to no value because most people don't want to learn it; even when they say they do. Test it out....do the two card monte (like David Blaine) and when they ask you how it's done, tell them. And watch their excitement deflate. When you give people value, the excitement should increase.
So don't put a trick on the back of your business card or even do a trick with the card. People keep your card because when they saw you perform, they were thinking about hiring you. If they are not thinking about hiring you then a trick will no make a difference.