The Piano Key is the image of a man reduced to a part of a mechanism, whose acts would be played from outside by the laws of nature. The underground man uses it to say what man would be if there were a formula for his will, and to defend the act that proves he is not a key.
In Chapter VII, the science of the future will teach man that he has no will and no caprice, that he is a piano key or an organ pin. In Chapter VIII, the narrator carries the hypothesis to its end: if one day the formula of our wills is found, man "will turn from a man into an organ pin or something of the sort," for what is a man without desires, without will, and without wishes?
The answer is the act without calculation. Man, even when covered in goods, would do some filth out of ingratitude and would desire the most destructive absurdity, to prove that he is a man: "he will convince himself that he is a man, and not a piano key!" If means are lacking, he will invent destruction, chaos, and suffering, and will launch a curse.
The image is the form of the narrator's fear: being predictable. Two readings: the heroism of refusal, or the irony of a man who affirms himself only when he loses. In Part Two the gesture returns in miniature: he decides not to pay Apollon's wages only to prove that he can, and he rehearses saying that he does not want to, "because I want it that way."