it is a resistance path next to a liquid crystal with image underneath. pushing the spot allows some current to flow generating heat which causes the liquid crystal to become transparent and show the image underneath in a proportional fashion.
Do they even still have those? I haven’t seen them on a battery in years. I liked having them, even back when they were just part of the packaging, but right on the battery, that was so convenient.
They seemed to disappear for a few years before I noticed they were selling batteries with them again (it’s the difference between Duracell Plus and Duracell Ultra, apparently. In the UK at least). That’s what prompted this topic.