When you view through your viewfinder and rotate the rim, you will see the amout of light coming in changes, tech it something abt screening off certain range of wavelength(if i am not wrong)
anywei the effect when you turn the rim should be one of the following depending on what you shooting:
cuts down reflections on surfaces
get clearer reflection on the water surfaces/water become more 'see through'
If yours is a stardard SLR cam, you should see the effect chnages when you rotate. For the white line i jus take it as a marking, maybe some pro can explain better =p
While looking through your viewfinder / LCD and rotating the outter ring of the filter, you'll notice a difference in the lighting of your subject as you rotate the ring. Basically, what the filter is doing is cutting out certain wavelengths of light while allowing others in. It adds more "punch" so to speak to colours.
BTW, the filter works best if your subject is at a 90deg. angle to the sun.