Theory of operation(most common): High current through the poorly conducting hardened bearing balls causes them to heat up and expand. Since they are precisly fitted between the inner and outer races, they are almost unable to expand in a radial direction way from the shaft and therefore theoretically expand in only two dimensions. this asymmetric expansion builds up stress, that tries to return the ball to an almost perfectly round geometry. By giving the shaft an initial spin (required) the balls rotate in the given direction and are capable of converting their deformation stress into a momentum causing a torque on the shaft. This concept will likely not work with any other heat source than electricity, since the balls need to be heated more aggresively than the races to keep the clearances in the correct relation. This can also be seen by the motor temporarely deccalerating and coming to a stop, whilst running again after a bit of cooldown time. Although of course severe permanent damage occurs to all parts of the circuit in relative motion to one another, causing more friction and more resistance. Thanks for watching.