A little fiddly to build, requires the cutting of short lengths of brass bar. Comes with lots of spares. Gears cut in acrylic for long life, nylon bushings on the shafts. Potentially an interesting model with an exposed mechanism that can be added to.
A build guide can be found here
The video below is an animated engineering drawing of the mechanism in action.
 
                         
         
         
         
         
         
         
        