Basically, the fusee drive is a means of evening out the power provided
by a spring motor; the motor is linked by a chain to a cone-shaped
spindle.  As the motor turns the chain winds from the barrel onto the
spindle and vice versa depending on whether it is being wound or is
running.

When the spring is tightly wound, it has less leverage on the spindle,
and so lets out less power, and as it unwinds it gains more and more
leverage until finally it has no resistance at all -- at which point,
if the system is properly designed, it has no power left anyway.  The
net result is a longer-lasting spring motor providing more even torque
throughout the winding.

regards,

Larry Smith