This quote from Powers seems really strange for me, especially the last sentence (italics by me):
These two outputs, if about equal, will cancel, leaving essentially no net output to affect the controlled quality. Certainly the net output cannot change as the “controlled” quantity changes in this region between the reference levels, since both outputs remain at maximum.
This means there is a range of values over which the controlled quantity cannot be protected against disturbance any more. Any moderate disturbance will change the controlled quantity,…
I think this should mean that if there is an arm wrestling and a tug of war going on between equally strong participants and the situation is frozen so that both participants pull or push with their full strength but the flag or clasped hands don’t move, then in that situation it should be possible for you (if you could do it invisibly) to move the hands or the flag easily back and forth. I really can’t believe this. Has anyone ever tried? There is some relevant discussion about stiffness in Martin’s PPC and I think it applies here. The more the participants use power the more difficult it should be to move the the hands or the flag, to disturb the controlled variable. The variable which is in a virtual reference state is protected from disturbances but not so much because the the controlling output would cancel the disturbances but because of the stiffness of the variable which is a consequence of the net output of the conflicting participants.
Does this make any sense?
Eetu