The spinning pair of
steel balls bobbing up and down on an
antique steam roller.
At rest the balls hang loose. As the motor turns, the balls spin and lift due to
centrifugal force, and in so doing restrict the flow of fuel or steam to the engine, slowing it down again. As it slows, so the balls fall, in comes more fuel and up they go again.
One of the few things I
remember from my
history lessons, along with "Boulton and
Watt's double acting
rotative steam engine".