Pete, that is fascinating I have to say. I, of course, am not performing on TV (Oh, the dreams, the dreams!) so my audiences don't have the same opportunity - and, truthfully, I wouldn't do this on TV anyway as it would be impossible to create the drama I want.
There may also be a transatlantic difference. TV audiences in particular are vastly different in the USA from what they are here. I think that this is also true, perhaps to a lesser extent, with live audiences.
Jon, your point about concealing that you know where the safe cup is touches upon the biggest difficulty I found when scripting this effect. It relies greatly upon the set up when I explain to the audience at the beginning how I want my two volunteers to behave.
One of them, the one who will set the spike and the cups up for me, is asked to remember what it feels like to make informed decisions. To think about what she's going to do, which holder will be used, which position the spike will be in and so on.
This is all nonsense of course, none of it matters except as a contrast to what I say to the other woman. She is asked to use her intuition, not to think about things, but to just react with her first impulse.
There's more to it than this, of course, I'm just giving the bare bones of the spiel here.
Now, when it comes to her answering the question "Is the spike under my hands?", the first time it doesn't matter - I'll either smash that cup if she says "No", or the other empty one if she says yes.
At this point, I slip in a little "escape" clause. I say something like "Now, things have changed a little. Sharon, here, might have suspected what I was going to do, but she wasn't certain. Now that she does know, it changes her psychological reaction slightly. She'll be worried that she might cause me to impale myself and that might interfere with her ability to go with her first impulse. So my job now, is to make a judgement about whether she's really using her intuition or not, so this time when you answer, I want you to look straight into my eyes...."
This is all rubbish of course, but it sounds good and gives me an out. However, I do want her to be looking into my eyes because of what happens next.
When I ask the second time "Is the spike under my hands?", it is, and I want her to say "Yes". In over 400 performances, the lady has said "No" I think about 6 times, because of another technique, and this is why I've asked her to look into my eyes.
She's looking straight into my eyes. As I ask the question, I'm nodding, almost imperceptibly. She almost always says "Yes" because of this. Try it, it works. I've never had anyone say they saw me nod, not even the lady herself. In fact, they nearly always say "It was weird, I just knew the spike was there".
On the very rare occasion that she says no, I use the out. I'll stand back and say "Did anyone else notice that she hesitated? You couldn't see it, but she blinked as well. That's a sign that she isn't going with her intuition. Let's try that again.". I'll then give it another go to try and get a "Yes".
If she still says no, I'll say "No, I still think you aren't letting go of your apprehension. Maybe I should rely on my own intuition". I stare at the two remaining cups for a while, I don't wave my hands over the cups as I stare, I just look and then smash the empty one, making an appropriate comment as I thank the two ladies.
I've just read all this back, and I know it reads a bit lame, but this is what I do and I swear that nobody ever says "Oh, you just knew where the spike was". On the contrary, I often get people saying that it's the best demonstration of intuition/NLP/Telepathy/psychic power that the've ever seen. Any explanation but the real one, so the empirical evidence is that it works for my audiences.
Someone else made the point about the "ghoulish" element to the audience's fascination, and I defintely agree with that. So much so that I often make a reference to it in the introduction - I compare it to the thousands who watch Formula 1 in the hope of seeing a good crash. That generally gets a laugh.
I don't think these effects are for everybody, and I do think that most people who do this kind of thing waste it, in the way I said before. I can only speak for my own experience, and I've no wish to gainsay anyone else's.
I can only repeat that I've done this over 400 times (including last night). I get the desired "No" followed by "Yes" almost always. There is only an infinitesimal chance of injury (I accept that I could have a brainstorm some day) and since it's my risk (I agree, I would never use the spectator's hand - that's just asking for it), I think that the drama it generates for me, and the reactions I get (it closes my cabaret show) justify that small risk.
I think that an awful lot of illusions would have to be ruled out if we became a "not even the slightest risk must be taken" profession.
Sorry this has been such a long post, I'm very enthusiastic about this effect, it's stood me in very good stead over time.