ian_s wrote:i thought the rules specified that they must have throttles. if thats the case i dont see any advantage to having them and not using them[...]
In part load conditions, it is usually more efficient to not throttling the engine but cut cylinders.
However, I don't know if really aggressive cylinder cutting could lead to unwanted resonances in the powertrain.
Perhaps they are just doing it the simple way, cutting three cylinders for torque demand <50% and then switching to 6 cylinders and contemporarily closing the throttle at torque demand >= 50%. And vice versa.
This is done in road cars too for lowering consumption, but usually by closing the the valves, which is not permitted in these engines:
5.9.2 Variable valve timing and variable valve lift profile systems are not permitted