The flex pin is used to avoid tilting of a planetary gear which is only fixed on one side in a carrier. A hollow pin is connected to the pin and their bending compensates in the ideal case. So three shafts are defined here: The pin, a hollow pin and the gear. In the graphics at the lower right you see the bending line of the shafts, dark blue is the gear on top.
The hollow pin is connected rigidly with the pin at two points to model the interference fit. Here experience from more accurate FEA is helpful to model the stiffness of the joint accuratly. But the differences between high and low stiffness can be evaluated with the shaft calculation very quick.