There's an article in this month's Mogmag about installing the engine and gearbox.
The chap who puts it all together says of the Centra drive that "...there are large rubber bushes which rotate when put under too much stress..."
There's also a picture of the unit being assembled and it looks as if the bushes could be compressed enough to allow some rotational slip. Might this be the cause of the occasional bang from the transmission?
I get it occasionaly, usually going from overun to picking up the throttle again at too low revs which could fit with the centra "slipping".
Adding to Chris and Dan's comments, I note that the Centa inner and outer rotors are described in the parts manual as
soft return. I wonder if this implies that slip as suggested is a design feature under extremes of torque.