This is a video cut from "Trick of The Mind" series aired in the UK. The clip here is taken directly from Season 1 -- Episode 1.
This is one of my favorite segments done by Derren. It explains for the most part how he did achieve the feat, and the trick is quite clever. Don't get too hung up on the ending though; it's merely a trick.
I've got all of Derren's work on my computer, and if you're interested in somehow obtaining the information, email me at firstname.lastname@example.org and we'll talk.