Hiya,
It seems to be using something that I've thought would make sense: a "clutch" between the pairs of cylinders, so that you can run off of just two when you don't need all the power of four cylinders.
I wonder if it can be scaled (way) down to have 35-55HP units? [Edit: I guess that they can!]
(click on image for link to very informative video)
This thing looks awesome -- it can be run with just 1 module, or with 2 when you need more power; as each module is fully balanced. It is very efficient --
about 40%, and has less heat output.