Pianotech

Watching the videos of PianoDisc systems I see that solenoid's plungers jump up out of coil when activated? I don't notice any reverting spring inside. Or does the plunger have a magnetic core at the bottom, not just a metal one? If the latter and no spring is used, then the plunger must be sucked in by the coil of an activated solenoid, not pushed up. And that was exactly what happening when I tested a PianoDisc solenoid I had got. So I'm bit perplexed. Could any one explained me the physics? 

I've never played with one of these before so I could be wrong, but the way I understand it is that the solenoid creates a magnetic field when you run current through the coil, and the metal cylinder/core is sucked into that field. So at rest, the core would be sitting somewhat below the level of the coil with the rod sticking up, and then when activated, the core jumps up so it's fully inside the coil and the rod pushes the back of the piano key up. I imagine it's some combination of gravity, the key weight, and maybe a spring underneath that brings the core back to rest position.

Thank you for the hint! It turned out that I inserted the plunger upside down! But Piano Disc solenoid has a black tube and it should contain metal core in deactivated state not vice versa. I just haven't paid attention to this, ha-ha)