There is an offset between center of circular grooves on each shaft and the clutch center (see upper picture). Each pink ball is positioned by the contact with grooves on both shafts and blue central ball. The latter can rotate around red pin.
With this arrangement, the plane containing ball centers always remains in a plane that bisects the angle α between the two shafts when α changes to meet condition of constant velocity.
STEP files of this video:
http://www.mediafire.com/file/kb4kbk5ra1473l
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