Voluntary, Involuntary, Revolution

According to Marx and Engels, activity in a Communist society is "voluntary", while activity in a Capitalist society is "natural", and, thus, by implication, involuntary. Now, separating a Capitalist society from a Communist society is, in their theory, Revolution, but whether or not that activity is voluntary seems undetermined. For example, if Revolution is conceived as a calculated means to a voluntary end, then it, too, is voluntary, while if it consists in mob hysteria, then it is not. Likewise, if it is an inevitable moment in a Dialectical process, it is Rational, not Natural, and, hence, voluntary, while as general History, in which individuals are caught up, it is involuntary. But, perhaps the conclusive factor has emerged in actual events--the recourse in some revolutions to totalitarian measures, which is an indication that Revolution is, in principle, not voluntary.