> I was under the impression that the rod is ridig, and therefore can't 
> twist (only rotate).

Nothing is perfectly rigid, a strain gauge on the rod will easily pick up 
the twist in a rod accurately enough to determine the torque and angle it is 
twisted.

On the older style mechanical systems they probably have a special small 
section that is a more flexible than the actual rod, that incorporates the 
valve design etc.

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