Pen barrel rotation
Introduction
Barrel rotation is very easy to understand. It's just a rotation of the pen along its long axis.
In the diagram below, barrel rotation is indicated by the arrows that go around the long axis of the pen (as indicated by the dotted line).

Benefits of barrel rotation
Barrel rotation is intended to help have brushes that mimic what traditional media brushes do - where rotating the brush around its long axis will rotate the brush on the canvas.
Popularity
I don't think barrel rotation is widely used, but absolutely some people have an art style that requires it. And for them, barrel rotation is critical.
Pens that support barrel rotation
Barrel rotation is a very rare feature on pens. I know of only three pens that support it.
Wacom Art Pen (KP-701E)
Wacom 6D Art Pen (ZP-600)
Apple Pencil Pro
Huawei M Pencil Pro
Keep in mind that these pens only work with specific tablets. You cannot buy these pens and assume they will work with the tablet you have.
Will barrel rotation every be adopted more widely?
We don't know. We don't have any information about other brands adopting barrel rotation.
Turning on barrel rotation
See: Using barrel rotation with your brush
Videos
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