Ricky Gevais: "Sorry sorry David seriously, this isn't a trick. You're just sticking a needle through your fucking arm. What are you doing? That's fucking horrible. You're a maniac. That's the worst thing I've ever seen in my life."
Still, I would tell everyone to be cautious believing a magician's explanation for any trick. Lying to mould people's memory of the trick is a really common technique. In one version of David Blaine's icepick through the hand[2] he says "This is the first time I let someone else choose the place" When he clearly chooses where to push the icepick through.
Remember: Magicians are allowed to lie. Sometimes it's just a simple thing like "I never touched the deck of cards" when actually they were holding them at the start of the trick. Other times it is fake explanation. "I am reading you using psychology" is a popular one with mentalists. Other people on a magicians TV show may lie too.
I don't know the particular David Blaine trick you are talking about, but I wouldn't be surprised if he does not actually chew on real glass.
There was a magician on "Penn & Teller: Fool Us" recently who told an interesting lie. His lie was that he was doing a trick.
His "trick" involved snatching the right card out of a fast falling stream of cards, with the stream going fast enough that it seems impossible beyond the ability of a human to use counting or timing to pinpoint the right card and reach in and grab it.
In fact, he had developed the speed and skill to do just that.
Here's a page from the magician showing how he does it: https://www.kostyakimlat.com/blog/fool-us-magic-trick-reveal...