The jump hour mechanism has a very unusual construction. Normally, you would just have a single tooth on the centre wheel that would advance a wheel with 12 teeth on the hour wheel every hour. That is of course not an entirely snappy affair, but how it’s done in other jump hour watches. This one has a spring holding the hour wheel in position (at the 8 o’clock marker) that engages with a tooth on the hour wheel. On the centre of the wheel, a spring builds up the tension on the hour wheel, which is released through a little notch on the minute wheel (at about 19 minutes in the previous photo). When the spring looses its tension with time, the change gets a bit sluggish, and there isn’t much you can do. The spring can’t really be made more powerful again.
I sweat a good three hours over the jump hour mechanism. If released too early, the hour wheel doesn’t change over quickly, and with the release a bit too late, the watch can jump two hours.