From previous googling during another cycle I learned there are two ways of doing this trick. One of which has zero risk but requires more mechanical skill, another has the risk evident here but is more mental and social in nature.
But perhaps there is only really one way to do it and another way many people misunderstand?
I have no interest in performing this trick.
Aww go on. All the cool kids are doing it! You’re not chicken are you?
Sounds like an industrial accident. Perhaps OSHA should have rules for magicians 
(And I wish we could just talk about the methods of this trick openly, the way we would about any other work accident, so that we could actually understand the risks and outcomes in an informed way.)
It’s all shits and giggles till somebody gets hurt.
Ta-Dah!
Well the social method is pretty simple, you just keep an eye on the person who moved the bags. They’re going to react when your hand moves over the one with the nail in it.
ZOMG - I LOVE the Amazing Johnathan. Finally saw him live in Vegas, front row, and he fucked with me!
He fucked with me!!!
That would depend on the volunteer remembering which is which
IIRC, it is a common trope for magicians (coughDerrenBrowncough) to falsely claim a social/psychological method for how they perform their illusion, one that makes their skill seem amazingly perceptive, using savant-like powers of observation, when in fact they are using a gimmick to perform the trick. The set up is part of what makes tricks seem remarkable rather than mundane.
Magicians, like actors and fiction writers, are part of a class of socially accepted, and often lauded, professional liars. For some reason, we, as humans, enjoy being lied to, but only in certain ways, and when well done.
There’s at least one performance where the magician actually stabbed the audience member’s hand while doing this trick. Not for the faint of heart…
(video shows two different failures for the trick, skip forward to 1:17 for the audience participation one)
Ahh!! I need a “Horrified” button to click on, not a heart button :-0
(And I’m guessing that most magicians aren’t the most solvent, liability insured people in the world, either.)
I seem to recall a magician on Penn and Teller’s “Fool Us” getting burned by the hosts for performing this trick with a young audience member- and, as I recall, they have very little patience for people who perform tricks that actually put a person in danger.
At least it was his hand.
2nd Story: My cousin got called up on stage for a trick like this- it involved a number of loaded/unloaded staple guns and the magician having her shuffle them and then fire the staples at his hand. The final two staplers (one of which is supposed to be loaded, the other not) are chosen between, and he had her hold the stapler to his head and pull the trigger. Which she did.
There was much blood.
Good for them! Er, unless they let the magician perform the act on their show and then castigated him for it. In which case they were a party to it, in spite of knowing better.
I put a sharp Phillips screwdriver through my hand once, palm to back, it hurt like a bugger but apart for the hole did no damage, I was very lucky, I pulled it out so quickly I thought I had just stabbed my palm until I saw the hole on the back.
Except I don’t understand how someone like David Blaine does it, when it’s a separate audience member picking the bag. Sure, Blaine could know which one it’s under the whole time, but if the audience member points to the bag with the spike, what does he do? “Ooops, nah, not going to do that one…”
As long as it’s a clean non-galvanized nail it’s not a big deal, you’ll hardly even have a scar. But having to pull a rusty dirty nail out through the bottom of your boot with a crowbar doth exceedingly suck.
Christ, what a hand hole.
Sadly, it was the latter. But they effectively ruined his trick afterwards.
EDIT: On further reflection, it seems totally possible the act was chosen by the producers of the show and that P&T had no prior knowledge. I have no evidence to support this- I merely offer it as an option. A hopeful one at that.
