Derren Brown's Svengali Hand-Stamp -- What's that all about?

A couple of Svengali attendees have posted pictures of their hand stamps. For example, see this one: Hand Stamp, which includes the caption "9889: Derren Brown Svengali - you have to be there to understand!"

What is the hand stamp all about? It's part of a trick, obviously. Does Derren select volunteers based on the number stamped on their hand? Or is something else going on here?

And here's another photograph of the stage, taken at the March 24 Liverpool show, apparently: Svengali Stage. The flickr caption calls this the "opening stage set," implying that the stage looks different later in the show.