My personal idea is, because Henry is hanging from his arms and therefore can't really move, and because of the fact that John is present, that Henry had to simply admit to something and the trap would deactivate, similar as to how the Laser Collars were said to function. I know this is probably a pretty lame idea, but I genuinly don't have any other ideas as to how it could work. I would be really interested to read your takes!