Labor-power is a virtual abstraction that does not exist in the real world. What exists concretely is engaged labor-power; in other words, effort. When labor-power loses its engagement, it “melts into thin air” (Marx), just like the once great Soviet Union.
Effort is engaged labor-power. What engages effort is an authority: authorization guarantees the effort’s usefulness. Its engagement incarnates a will that may ground such authorization by supporting its use or not. The actual embodiment of the effort marks its body, the material place of its existence. What (re)organizes its engagement is a system that may ground such embodiment by supporting its existence or not.
Capitalism is the system in which “capital” names the authority that engages the efforts of the people (workers) by supplying them wages, i.e. by giving them a minimal portion of its authority to engage the efforts of other people. Capital is useful for the workers insofar as it gives them the authority to buy stuff in the market. But above all, capital is useful to itself: through the efforts of the body of workers, capital gradually strengthens its existing authority by accumulating surplus-value. Thereby capital (re)organizes the workers’ engagement as a system, and that’s why we call it capitalism. Capital’s authority ultimately rests on the free will of the workers: “it works” only insofar as “they work”, i.e. insofar as they voluntarily “sell their labor-power” and engage their efforts inside the system. After all, the material existence of the capitalist system solely consists of the bodies of workers that it supports and engages in itself.
In communism, what engages the efforts of the people is instead the authority of the communist party. The actual embodiment of these efforts is called the “working class” (in relation to the minimal engagement of the workers) or the “proletariat” (in relation to the maximal engagement of the communists). The party is useful to the people insofar as it “socializes the means of production”, i.e. it makes the economy function (with centralization and without markets in traditional communism). But above all, the party is useful to itself: through the efforts of the communists, the party fortifies its authority by accumulating surplus-enjoyment: what it really centralizes is the people’s libidinal economy. Thereby the communist party (re)organizes the workers’ and communists’ engagement as a system, and that’s how it’s called communism. The party’s authority ultimately rests on the free will of the workers and the communists: “it works” only insofar as “they work”, i.e. insofar as their efforts are engaged inside the communist system. After all, the material existence of the communist system solely consists of the bodies of workers and communists that it supports and engages in itself.
This simple analysis is applicable to any system that (re)organizes the efforts of a group of people. It is based on the diagram called “Postmodern Alienation Model”  which provides a “real abstraction” (Sohn-Rethel) whose terms remain the same for any system being analyzed. The terms of this conception is based on a Žižekian understanding of Lacan’s four discourses: Authorization denotes the function of the Master-Signifier S1, embodiment denotes the function of the objet a, will denotes the function of the barred subject $, and system denotes the function of knowledge S2 .
 See “Postmodern Alienation Model”
 See “What Makes a Symbolic Order?”