I cannot speak about any piano, because I had chance to tinker with only one, but that piano had springs. Keys itself return into their normal position because

I cannot speak about any piano, because I had chance to tinker with only one, but that piano had springs. Keys itself return into their normal position because of gravity, but part of key action mechanism was powered by a spring. I discovered it while trying to fix some keys which was too slow to "recharge", to return to a normal position after a soft note, so I was unable to make a sequence of a short duration soft notes from the same key. I fixed it by stretching springs long enough for a plastic deformation to occur. Loud notes worked ok, because their "recharge" was driven fully by springness of the strings. Though it was just one piano, I do not know how different the mechanics can be between different pianos.