AFAIK it works by analyzing your docker image, checking whats actually used and then throwing out anything else.
For example if you use the Ubuntu base image you have a full minimal OS install. If you’re now running a python server for example it’s highly unlikely that you will need the perl interpreter that’s in the default install so it can be thrown out.
It can get problematic if you want to run something that loads libraries or runs programs dynamically at runtime, since the tool can’t easily detect them then and you need to manually intervene. Tried it once on a custom machine learning container and it kept throwing out parts that I actually needed, so I gave up in the end.
It’s usefulness is also somewhat limited, since docker containers also share their base images. So if you have three containers running that are all based on Ubuntu 22.04 you will still only have to download it once
I can’t find them at any of the German retailers at least. Only one that claims it will probably be sold by 23.10