Algodoo

Very, very strange. I cannot fathom what's causing that.

(Warning! Science Content!) well, since the hinges stretch, at the beginning, the spring gets it moving-the hinges get stretched. Since the weight on the end of the "rotor" has a lot of inertia, due to it's high mass, it keeps one hinge stretched more than the other. the smaller one has more pulling force, and acts like a spring and pulls the arm down, then the other gets smaller, and the whole things just keeps running over and over again. (I watched the whole thing in slo mo to figure that out)