Double Cardan joints allow to adjust angle A between input and output shafts even during rotary transmission. This case shows +/- 90 deg regulation and proves that double Cardan joints are of constant velocity.
Due to the gear planetary drive of two gear sectors and orange crank, angle between input (or output) shaft and the yellow intermediate shaft is always equal to A/2.
The pin axles on the yellow-violet intermediate shaft must be parallel each other.