Here is some reading: Curvature - Wikipedia
But basically the twisted curvature we see is the normal of the smallest fitted sphere rolled around a point on the 3D-spline. The normal is intersecting the point of the spline and the center of the sphere. The size of the comb is the inverse of the radius with a scale/ratio and is translated on the opposite side of the spline.
So: Yes, the curvature is twisting.
This is also quite interesting: Curvature - Wikipedia
Even if the start and end point of a spline is the same, they might have different curvature. Don’t know if it is applicable to our surfaces and splines.