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)