Side view of shoulder.  Many tradeoffs were
made to keep the shoulder simple, while still
providing most, but not all, of the range of motions
of a human shoulder.  In this picture, when the
servo spins the arm (bolt on bottom) comes
out toward you on it's way up.  The U-Joint
is constrained to only allow this motion; the
other U-Joint motion is immovable by tightening
the bolt (not visible here) into the square block
in the center of the U-Joint.  When the hub
spins, this entire assembly spins with it.