The truth is, there were two guys named Doug and Dave who figured out how to use a plank and some rope to beautify the fields at night, having drawn out their "mystical" shapes in the local pub. Kind of a disappointment, but also a joy to meet these agricultural artists.
Circles have a mystery all of their own because a true circle has no ending and no beginning. In mathematical size, it includes 360 degrees from any point on its circumference, and all points are equally distant from its center.
The wrist actually does have boundaries that are discernible. There are 8 little bones in two crooked lines that make up the wrist. They fill in the space between the bones of the hand and the two bones of the forearm. Each bone has a unique shape and a unique Latin name that relates to its shape. The wrist is quite beautiful in its own way.
But it is not a circle, nor does it describe one as it moves through space, earthly space, piano playing space. The wrist bones allow the hand to move in many directions, but none of them is a circle. A segment of a circle is an arc. Allowing the hand to drop from the wrist creates a visible arc through the bones of the wrist, especially when viewed from above, but the arc can not continue into a circle.
Like the hand, the wrist does not have one ideal position for piano playing. Encouraging a flexible wrist helps pianists access the intricate movements of the wrist bones. Celebrate these movements the next time you lift a pint in honor of Doug and Dave.