They are electronically controlling valves in a hydraulic pump to switch cylinders into and out of service.
There is an animation and a good description here:
There is an animation and a good description here:
So as I understand it, their system allows one to disconnect volume and pressure of the fluid coming out of the pump, with the actual speed at which the pump is operating. That lets you vary the work done by the hydraulic motor independently of the speed of the pump without resorting to a transmission.
Given that they have it running on a 7MW turbine that is pretty cool. (ok a 7MW wind turbine is pretty cool all by itself)
It would seem like maintaining the valve system and controller would be easier than maintaining a gearbox (they say as much in their overview) but I didn't find any good operational metrics (is it 100 hrs a month of maintenance vs 10 hrs? Or 60hr of maintenence for every kWh vs 6hr? something like that)