Mike:
Consider this. A standard U-joint will generate a 2 per rev vibration once the angular difference comes off of 90*. The angular difference causes an increase and 180* later a decrease in rpm on the output side of the joint.
To solve this problem. add a short shaft and a second U-joint. Now you essentially have a doubled 2 per rev vibration, but the angular difference are equal to each other 180* apart. So the end result is a cancellation of the 2 per rev vibration.
Or, think about it this way:
your rotor mounted on a single U-joint would require a lead-lag hinge to compensate for increase and decrease of rpm difference between the rotor hub and driving shaft. If you add a short shaft and a second U-joint (CV joint) then the rpm change would be cancelled out within the joint and the driving shaft and rotor hub would not have a 2 per rev vibration and no need for lead-lag hinge
This is the same principle applied to a front wheel drive automobile. A CV joint takes care of the problem. If you ever drove a 4 wheel drive vehicle on asphalt and turn the steering wheel. You can feel a side to side pull within the steering wheel. That is the 2 per rev vibration back into the steering controls because the tire (rotor) cannot slip. If you build the drive joint as a CV joint the 2 per rev vibration is cancelled out. Hence driving a normal front wheel driven vehicle in a turn - smooth.
Hopefully this will allow you to visualize this problem.