The PlayStation drew a picture on a triangle by stretching it straight across the screen, without correcting for depth. That is "affine" mapping, and it is why the floors of the time bent and swam as the camera moved. affine: true on a material (or on createModel and createMesh) is exactly that.
The warp grows with how big a triangle is on screen. The left half of this corridor is made of whole squares and swims; the right half is the same squares cut into eight by eight and barely moves. Cutting was what the games of the time did to keep it down. Press space to switch it off and see the picture held true.
If you write a material shader of your own, its fragment receives the uv already stretched when the material is affine, so it works there too.