Knees are seen in exponential curves. Exponential curves show up in all kinds of situations, where you have a sort of compounding change happening, like population growth. What happens is growth seems slow at first, then it takes off like crazy. A sort of informal word for the area when it goes from slow to fast is the 'knee.'
The idea I was suggesting to Finn was that the displacement of workers by machines has been building for a while, but slow growth in the past doesn't mean it won't get real fast in a short time.
Picture shows a mark at the middle of the 'knee' of an exponential curve:
