OK, so it is:
OPTIONS > GENERAL > ACCESSIBILITY and about half way down. COCKPIT INTERACTION SYSTEM is the 3rd one down under USER INTERFACE heading:
T
With this set to LEGACY I can left-click on the left or right side of dials to turn them left or right and can put the mouse over the middle of the dial and use the scroll wheel to turn them left or right.
With this set to LOCK I move the mouse over the dial and it turns blue, then I click and drag left or right to turn the dial left or right. However, hovering over the dial and using the mouse scroll wheel does also turn the dial
I’m testing this in the TBM 930