I can't just cut the power at the end of travel if I'm using a spring mechanism to return to the original position (which I will do, because it's more consistent with the way many keyed wind instruments work, e.g. since the valves on a trumpet already have springs in them, it would be redundant engineering to remove the springs just to be required to replace them with a powered motor or solenoid for the return trip). If I did, the finger would no longer have it's pressing power, but as I wrote earlier, I could lock the finger in it's press position, and presuming it's compressed some sort of sponge against the key, hole, or valve, it will continue to do so until unlocked. The compression aspect is most important in the instance where the finger is covering a hole, since in reality that requires squishing the flesh of your fingers to completely seal the hole.