Watching ballet dancers, I can be convinced that they have elbows unlike mine. Their elbows are not pointy or edgy or rough but seamlessly fluid, almost as though they have no bony structure at all. Dancers achieve this artistic skill through disciplined movements of the arms, a series known as port de bras, translated as "carriage of the arms".
What dancers learn is that each joint of the arm has a particular range of motion, thanks to its shape and attachments. The elbow can flex, extend, pronate and supinate. The upper arm joint is the most freely moving joint in the body, and the joint where the collarbone meets the breast bone has small but critical horizontal and rotational movements. These movements, in conjunction with wrist movements, create the flowing sequence of port de bras.
Just as the well-trained dancer deceives me, pianists can deceive me into thinking I am seeing movements that are not humanly possible. As explained in the previous post, there is no fully circular movement potential in the structure of the wrist. But the upper arm does have circular potential. Put this in combination with the semi-circular rotation at the elbow, and it is a short leap of faith to translate these movements down to the wrist.
The problem with this optical illusion is that pianists may attempt to make movements with the wrist that happen in other joints of the arm. This is both limiting and potentially dangerous. Because the nerves that run the fingers move through the carpal arch on the underside of the wrist, putting stress on the wrist can compress these nerves. Potential outcomes are pain, numbness, tingling, and limited range of motion.
If you know the ranges of motion for the four main joints of the arm, you will be able to determine the actual movement from the perceived movement. With that information, you will be "armed" to create artistic illusions without fear of injury.