Two arms...

[iacob alex]

...let's say a continuous variable/oscillatory leverage , with two masses ( stabile+mobile...) , playing gravity and inertia , at :  www.youtube.com/watch?v=N6G5aC9BtSc
You can take it as a suggestion , so to replace the circus performer with the most simple " thing "...and obtain self-running motion.
     Al_ex

[ARMCORTEX]

If you had, this motor, mechanically coupled to the output.

You would be making a positive feedback loop.

Eventually it would break if left like this , that is a certainty.

You can also reflect back the weight inwards, but that sytem would geat a beating.

[iacob alex]

.....as an experiment that is kept secret , at :
     https://m.youtube.com/watch?v=cTdr_MDTtts
     Al_ex