The energy stored in the (inevitably) slightly elastic string by the act of plucking must go somewhere and the "path of least resistance" is sought by the rotor as default, rather like water running downhill.

The shorter length, to one side of the rotor will initially act as a complete string just as the longer section will but being subject to the same tension will vibrate faster and impart more "nudges" per second to the rotor, thus edging it nearer to the central point of equilibrium, allowing the resonant frequency of each section to match, in turn allowing optimum dissipation of the potential energy of the whole string.

This demonstrates just exactly what an efficient machine a music string is. Quite possibly, by definition, a music string is the most efficient machine, being a near perfect illustration of the shortest distance between two points in a curved universe. What could be simpler?

Maybe one could compare the beauty of this deceptive simplicity to Quantum itself. At the very least, this small experiment demonstrates that the dynamics of a vibrating music string involve circular, ellipical or more complex patterns of movement (otherwise no spinning of the rotor would occur).