Here's how the spike trick is supposed to work: a magician shows the spectator a large nail mounted on a block of wood. He sets it on the table so the nail is pointed up. Then he covers the nail with a paper bag. He places three identical paper bags next to the bag covering the nail. He turns his back and asks the spectator to shuffle the four bags around on the table so that the magician has no idea which one has the nail. The magician turns around to face the bags, then slams his hand down one a bag. It was empty. He repeats the process until only one bag is left. He lifts the bag to reveal the nail. It's a nerve-wracking trick.
Recently a magician performed the trick and made a bad mistake, driving the nail through his hand. You can see the photos here. Fortunately, he's going to recover.
And here's a video from 2007 that captures another magician stabbing himself. It's not too graphic, but it is hard to watch anyway because you know what's coming.
I have no interest in performing this trick.