Thank you very much for your nice words! I appreciate your review.
That's said, I'm blocked at level 8, where I can't undestand why the power comes from 20 to 0 all of a sudden
I can see how it might be confusing, because the gear keeps rotating - it probably shouldn't. However, in the manual section of the game page you can read more about the mechanics:
If there is a shaft driving the gear, it will also transfer the torque to another shaft mounted in the same direction.
Since the gear is being driven from the bottom, the only shaft the torque can be transferred to is the one mounted from the top. Hopefully that helps!