That aint the joke - the idea is that DB is a smooth psychological illusionist, and is poised to show us his latest trickery...
However, his latest piece of smoothness is in fact a very innapropriate and simple ploy that anyone could manage. The humour is compounded by the fact that he is rather smug about his ploy, despite having only managed to blag himself a trip to the rubbish dump for an amount of cash that anyone would have to pay
Whats even more hilarious is that Derren is pulling those mystical faces and hand positions in every square he is in...