Variable torque/force was experimented with by a Japanese company a few years ago by a "shiftless transmission". It was mechanically simple, involving 2 long, slender cone shaped shafts, one over the other, and pointing in opposite directions. They were connected by an elastic belt that was freely allowed to slide left or right, depending on the torque required. This had the effect of changing gears, except the "gearing" was infinitely variable by the smoothly changing diameters of a cone-shaped shaft.
The coned sections weren't straight-edged, but rather very shallow parabolic curves. This made the belt stay centered at a specific point along the length (the analog being a specific gear-ratio). If my memory serves, as more or less torque was needed, the angle between the 2 opposing conical shafts would change just enough to cause the belt to find the path of least resistance: the minimal distance. That's why they were lathed in the shape of a parabola.
I would have been interested to know what ever came from (or became of) these trials.
Brian Jackson