From what I could gather, it's a common parlour trick in his circles to pour liquid nitrogen into your mouth, at which point it will sort of stay on your tongue

From what I could gather, it's a common parlour trick in his circles to pour liquid nitrogen into your mouth, at which point it will sort of stay on your tongue (above an air cushion that protects the tissue) until it's evaporated. But this time he swallowed, and once it's inside your body the vapour cannot escape into the room as it expands, creating horrific amounts of pressure on your insides.