Just push the piston all the way in, and the rubber clicks when it comes out the other end of the cylinder. This way you can store it in a compact package.
Interesting. I’m using the AP Go, so maybe the normal one doesn’t extend all the way through.
Edit: just checked how my normal AP works. It’s basically the same as the AP Go, but I guess this is a fairly new model, so there may have been some changes.
Are you using an older version?