A Driven Part has external references to the Driving Part. When you change features of the Driven Part that are associated with the Driven Part, the Driven Part will also update.
For a real world example, take a rubber band and place it around your 4 fingers. Consider your hand to be the Driving Part, and the rubber band to be the Driven Part. As you spread your fingers apart, the rubber band changes shape, it is being "driven" by the distance between your fingers.
I hope that helps.
Ray Reynolds
"Computers in the future may weigh no more than 1.5 tons."
Popular Mechanics, forecasting the relentless march of science, 1949
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