I am quoting from a book by John Fay,"The helicopter and how it flies" Test Pilot and Instructor, Westland Helicopters 1967,
"Hookes Joint Effect, The inclination of the rotor disc is obtained by the blades moving up and down about flapping hinges during rotation. Thus although the rotor disc is inclined, the drive shaft remains fixed in the fuselage. When the rotor disc is inclined at any angle other than that normal to the drive shaft, the blades tend to move on their drag hinges in order to maintain constant speed. In the case of a hovering helicopter, theblades are at 90 degrees to each other with respect to the plane parallel to the plane of the hub.
When the rotor disc is tilted by blade flapping, the two athwartship blades, in order to maintain a constant velocity in the plane of rotation, must move on their drag hinges to the position shown. If drag hinges were not fitted, the blades would be forced to accelerate and decelerate with every revolution.
The situation occuring when the plane of the rotor disc and the drive shaft are not normal to each other is often referred to as "Hookes joint effect". A hookes joint is a universal joint, but is not a constant velocity joint. Thus when the two shafts, with the joint in between, are not in line, although one shaft is rotating at constant speed, the other is rotating in a series of accelerations and decelerations."
End of Quote.....
Hope this helps, cant get much better than that i guess.