It is an application of double Cardano joints.
The joint is shown at:
https://youtu.be/95bZU6ylEzw
The direction of upper disk is kept unchanged and its surface is kept always horizontal.
Each point of the disk moves on a spherical surface.
See a similar mechanism where the upper disk can be controlled by grounded motors:
http://youtu.be/iQ5TkU04Xdc
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