I wouldn’t advise anyone to try and emulate this trick. Not only is it difficult squeezing your head through the neck of the jar, it’s also incredibly hot inside. The water is designed to keep things cool, but that in itself adds to the danger. It’s impossible to breathe.
And if you think it’s a squeeze getting your head in there, wait until you try and get it out! No amount of jello or washing up liquid is going to get your bony skull back out that 2 inch wide neck. You may want someone close by with a big hammer, and even that poses its own problems. Don’t forget too, as you’re struggling, you can’t breathe, there’s no where for the ice cold water to go and you’re gagging to take a breath. What will you do?
Yup, this isn’t for the feint hearted, so if you do happen to come across one of these old demi-johns, then my advice is, Don’t Try This At Home.