Polarising Filters & Their Correct Use.
Many people use the polarising filter incorrectly by reading the exposure through the filter when it is displaying its greatest effect. By doing that, they lose the darkening of the sky or foliage because the meter then compensates by increasing the exposure (ie what was a dark sky becomes a light sky again on the film, and the highlights get even lighter than they were).
A portion of the scene not affected by the polarising effect has the same EV regardless of the rotation of the filter. Rotating the filter selectively removes polarised light from the scene, giving a saturated rendition of the colours in the scene. By then increasing the exposure when taking the picture, you undo the main effect that you were after.
The best way I've found to use a polarising filter is to measure the filter factor with an incedent meter (flat disk) and apply that factor when calculating exposure. To use this method, use a scene dominated by non-polarised light, such as an overcast sky, a studio strobe or an indoor scene to compare the EV with and without the filter. That is the filter factor for that filter, which you should always apply to your meter reading.The filter factor on my polarisers is a constant 1.6 stops. It is the effect of polarised light being filtered that gives the impression of a variable filter factor.
The next best way (if you are on the fly and using TTL metering) is to set the exposure with the filter showing minimal filtration and then "dial up" the required effect without altering the exposure value.