Post-structuralism is grounded in the concept of overdetermination, even when the concept does not appear explicitly in textual presentations.

Overdetermination as an epistemology implies the absence of a break between discourse and the objects of discourse. It implies that theory is not separate from reality nor is reality separate from theory. We SEE what we are TAUGHT to see in the concepts we LEARN. The concepts we LEARN are complexly shaped by the stream of reality of which we are elements.

Post-structuralism recognizes the power of discourse to shape reality (both perceptions of reality and the concrete reality that is perceived). Discourse (theory) can produce SIGHT of FICTIVE objects, such as race (as in white race), or deny SIGHT of REAL social relationships/objects, such as class (as in feudal class relationships). In other words, at any given moment and theoretical understanding, we experience only limited aspects of the world and some of what we experience is based on falsehoods embedded in some of the discourses we have learned (falsehoods in the sense of not existing separately from the theoretical constructs, not even satisfying the coherence of defined objects within that discourse, as subject to investigation on the basis of the internal rules of coherence and fact of the discourse (e.g. the genetic notion of race fails upon inspection of the correlation between those physical features ascribed to races and the genetic make-up of those so grouped)).

As an ontology, overdetermination implies that existence is comprised of mutually constitutive processes.  This overdetermined existence/BEING is complex and not conducive to the rigidity of the grand narrative which seeks to find a singular explicable Truth about reality. In this complexity, all processes are continuously in a state of transformation and Processes are continuous movement/change/happening.   

As Heraclitus said, "You can never step in the same river twice."

Post-structuralism means to go beyond the structuralism of theories that imply a rigid inner logic to relationships that describe any aspect of social reality, whether in language (Ferdinand de Saussure or, more recently, Noam Chomsky) or in economics (orthodox Marxism, neoclassicalism, or Keynesianism). Marx and Freud have, alternatively, been described as structuralists (creators of deterministic grand narratives) and as post-structuralists (breaking with the enterprise of creating deterministic grand narratives) in their theoretical innovations and inventions. The same has been the case with Michel Foucault and Louis Althusser, although both are more generally regarded as post-structuralist thinkers (albeit having had their structuralist moments).

Is the totality like a machine with different parts or a river that is forever moving and changing, never the same? Are we living in the universe of Parmenides or the uni(multi)verse of Heraclitus?

Post-structuralism was/is itself overdetermined by social processes, including the student movements of the 1960s for democracy, nonexploitative economic processes, and nonoppressive social relationships, in general, and against the economic determinism of orthodox (read Stalinist) versions of Marxism.  The theorists who innovated the logic of overdeterminism (first introduced into social science by Freud at the turn of the twentieth century) wanted to displace notions of static social structures with notions of social processes that were always in flux and, therefore, capable of being changed in progressive ways (or, alternatively, changed in regressive ways).  It was preeminently an attack on apathy.  

The 1968 student uprisings in Paris and Prague  ("Prague Spring")  were important moments that fed the general movement to rethink the dynamics by which history is made.

In the post-structuralist world, theory necessarily has an effect, a complex, ripple-like effect on EVERYTHING.

The movie, Memento, is an interesting example of cinematic post-structuralism.

See Barbara Epstein's excellent and thought provoking, " Interpreting the World (Without Necessarily Changing It)"