Miss Piggy - I can't give too much away as Derren himself asks people not to and others on this forum may yet still intend going to one of his shows. Suffice to say that if you've seen the TV shows, you'll have an idea of what he's up to but it's much better to see it done live where you can see for certain that there's no camera trickery or such like.
One of his victims (volunteers?) was physically sick after being 'ordered' on stage (although this was probably nerves as even Derren seemed surprised by this) and others were clearly severly shaken by the experience.
Gaff - Sorry, you're right - I've switched PM on now