I think it appeals to people's ego and need to fit in and be accepted/approved. I observed that he said that it would work on people who are well-balanced and creative and intelligent. People will want to see themselves in that category - if they don't stick, then the implication is that they are the unbalanced, uncreative, thick ones! Personally, I have no interest in being part of a group of people whom Derren Brown defines as well-balanced, creative and intelligent. So it didn't work on me, because I didn't have that motivation. Also, logically, I realised that in order to stand up, you have to move your head outside of the imaginary circle - you need to lean forward significantly to stand up. So I didn't keep my head in the circle that he asked us to keep our heads in.