Looks like the derivative of the angle is continuous.
When you drive around a street corner in a car, you start by turning the wheel to turn a little, then more, then less, then you drive straight again. This looks like that kind of curve.
It's subtle. Zoom to the middle of the sample image and compare (the vertical part of) the curve of the two top corners. You will notice like... 5 pixels of difference, then the curve gets aliased in the smooth version. I would like to see it in an actual site though.
diff: https://github.com/user-attachments/assets/3edb0bbd-4989-442...