Timothy Walters Kleinert
So here's the first possibility---in that moment when you shift your "intent", you might be shifting around various internal structures, such that the main, err... "avenue" of force is changed. I think this is what happens in the "center"/dantien/tanden. Certain internal structures are manipulated to induce general movements of expansion and condensing. But this may also happen on a smaller scale throughout the body, who knows.
Potentiating the fascial stuctures by passive means (hormonal or otherwise) does do,one thing that specifically distinguishes it from voluntary muscular contraction -- is that it pre-potentiates the action of reflex arcs, triggered by passive stretch of the tendons. The Jendrassik Maneuver reflex test most simply demonstrates this. This has also been shown to evoke involuntary stepping motions iun the lower body from upper body passive stretch potentials that are not evoked when the passive tendon stretch is not present.