May 29, 2018:
"The Materialist Dialectic" is not Hegel's idealist dialectic inverted. In Hegel, the dialectic is the logic of a simple binary contradiction whose evolution is internally conditioned, that is, whose outcome is teleologically present in its origin. Althusser argues that Marx's famous figure of "inverting" the Hegelian dialectic is ambiguous and inadequate, for if taken literally it would leave intact the simple binary contradiction, the teleology, and the end which is already inscribed in its origin. He demonstrates that the materialist dialectic operative although not conceptualized in mature Marxism, is different, based on complexly articulated nexuses of contradictions whose overdetermination results in "ruptural unities" that are "explosively conditioned". You can think of dialectics in Marx as something like tectonics in geology: pressures mount from contrary directions until the weak point — the most overdetermined point of intersection of forces — suddenly breaks. That's my analogy, not Althusser's; and we know from Althusser to be suspicious of metaphors and analogies as indexes of nonexistent concepts. But, they help you to "see" the idea — another metaphor — so there you go.
More: in Marx this dialectic which is not the function of an end is always tendential, meaning, existing in overdetermined relationships with simultaneous counter-tendencies which have the capability of holding back, or undoing, or defeating the tendency under consideration. You know this from Capital: the law of the tendency of the rate of profit to fall. All "laws" in Capital are tendential. Althusser describes this as "a dialectic of the tendency". Althusser's Marxism rejects the Second International, with its invariant succession of world-historical modes of production conceived on the model of and with the logic of Hegel's successive epochs of the world spirit — a teleology in which the eventual victory of socialism is inevitable. For Althusser, it's always possible to lose.