It may be a term Nesbit invented I don't know but that doesn't matter.
Think of a wagon wheel. YOu have a rim and you have spokes extending to the hub (which has a certain radius). Now look at figure 2 of : http://www.motionanalysis.com/pdf/2005_nesbit.pdf . Those are diagrams of golfer swings (Think of each club freezeframe as a spoke with the head on the "rim" and the grip end on the "hub".
So the Hub is the "path" your hands take from the top down to and beyond the ball. The "instantaneous" radius of that path is what matters. You want it to decrease (This is what prevents casting and maximizes club head speed). If you fit a curve to that hand path of a better golfer it would look like a contracting spiral.
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