I don't know how they do it either, but I can assure you they are able to do so. It's not personal experience, I don't wear a chain, or any jewelry for that matter, however I have friends who have been the victims of similar events and I was present once when it happened, under similar circumstances to this incident, my mate didn't even realise immediately and I certainly didn't see it happen. They are very good at it. Practice makes perfect and they certainly do that.