You are in a train.
The nearby tracks are a blur.
If you try to make a sharp picture of them you really need to be fast. The camera needs to turn sharp around its vertical ax to follow the tracks*.
So your picture includes a bit of the front, the side and the back of the track.
As the camera turns you, in fact, turn around the object. This causes a vortex in the blur around the tracks.
You still move not fast enough to sharpen the foreground but to fast to sharpen the background. So the vortex contains 3D information just like DOF**.
This makes the pan vortex fascinating
Space by speed.

* This is called: panning
**Depth of field ( the focus field of a camara)

What do you need?

Apart from the camera, obviously, I like to use a small handle, fixed as tripod, with a revolving grip.

What are the setting on the camera?

The camera’s software does not like it when everything is a blur.
It can’t get a grip on the situation so it is best if you do it yourself.
Manuel focus and exposure solves that.
Image stabilization off.

I use a RX100
It’s relatively small sensor gives al lot of sharpness when stopped down.

Manual, speed usually 1/20 or 1/30
Manual focus near ∞
Aperture between 8 and 11
Low ISO

What goes wrong?

Depending the situation:

1. Coordination: pressing the shutter when moving fast at exactly the right moment is not easy.
2. You get the speed wrong.
3. A bumpy track.
4. Lights changes very fast.
5. Reflections, dirty windows, all things like that.

You really want to know more?

The pan vortex is one of the little known optical effects in photography.
Here, in short, is what happens:
You try to compensate the strait movement of the train with a circular movement of your camera.
If you focus on the tracks you tilt the camera downwards. A pan along the vertical axis of your camera means that your camera is only horizontal in the middle of that pan.

As long as you take your pictures at a strait angle to the tracks it more or less works out. (Fig 1-A)
There is some crescent moon curved plane that is more or less sharp, horizontal shapes and vertical shapes reacts very differently, and the blur over the vortex bends inwards.
If you make a pan using the world’s vertical axis, the crescent moon shape and the bending disappears
This is unpractical :(Fig 1-B) You need to keep the camera horizontal wile following with your subject.
But it shows were the distortions come from.