Derren Brown is an entertainer. Notice the constant change of camera angle? That means the film was edited to show you what they want to show you. I also suggest you read Derren's book. He explains, it's magic tricks. In fact Penguin Magic has a book on the subject that teaches magicians how to play this trick.
I've never read the how to book but I do know one aspect of the trick is reading the reflection on the glasses.
The reading aspect aside, Derren is a brilliant entertainer and has a wonderful understanding of human behaviour. So he's able to prime his subjects to obtain the outcome he wants. So if you want to entertain people the way Derren does, I suggest following his path. PhotoReading has a more practical use.
Alex
