For those who want a deep dive into polarized light:
https://en.wikipedia.org/wiki/Polarization_(waves)
Light that reflects off glass or water is polarized one way. Polarized sun glasses are polarized so that they block light that is reflected off a pool of water at a shallow angle. Since most windshields are at an angle what will generate the same polarization as a pool of water (cars all facing the same direction) polarized sun glasses will also reduce the reflections from windshields.
In this specific case, the wind shield is curved. so only one part of the windshield can have its glare reflections reduced. If you change the rotation of the CFL lens, the place on the windscreen where the reflection is reduced will move to a different location.
Most will want the CFL so that windscreen reflections from other cars are reduced, so they want the polarization the same as that of polarized sun glasses. This will result in the light being blocked completely when light first passes through the sun glasses and then through the CFL held at 90 degrees to the way it is mounted on the camera.
Why CFL (circularly polarized lens)? All of the above is a discussion of linear polarized light. Some cameras have a problem with polarized light. A CFL is a linear polarizer with a quarter wave plate. The quarter wave plate is made of a material that has a different index of refraction for different polarizations of light. The thickness of the quarter wave plate is such that it delays one polarization 1/4 of the wavelength of light. This is mounted such that the axis of the quarter wave plates polarization dependant index of refraction difference is at 45 degrees to the linear polarizer. This means that half of the polarized light is delayed by 90 degrees in phase, and you get a rotating electromagnetic wave. The cameras that are sensitive to polarized light don't see rotationaly polarized light as being polarized, so they are happy. The CFL still works to block light of a particular polarization when that light comes through the linear polarizer first.
Do not trust an LCD screen as a source for polarized light unless you know for certain that it is not a passive 3D TV screen. Passive 3D TV screens use rotational polarizers to alternately polarize rows of pixels either clockwise or counter clockwise. This allows the circular polarizers in 3D glasses to only allow the correct image to be seen by the correct eye. If you look at the screen through a linear polarizer, both images will get through, and rotating the linear polarizer would have no effect.