No, that\'s not what it does. It passes the full amount of all the present frequencies below the cutoff point, and it attenuates (cuts) progressively more from progressively higher frequencies. Different filters have different slopes of attenuation, so a mild filter will offer 12 decibels of attenuation for every octave over the cutoff point. A more radical filter will offer 24 db/octave, and you can run filters in series to get steeper rates of attenuation.
When you modulate a filter, you can\'t change the slope of frequency attenuation, but you change the point at which the attenuation begins.
(I\'m ignoring the effects of resonance, where a variable feedback loop emphases (increases) frequencies at and around the cutoff point, that provides a very distinctive and unnatural sound as the emphasis point moves across harmonics)
I\'m sure there\'s good info on stuff like this on the web, I\'d try a yahoo lookup on \'lowpass filter tutorial resonance\' or something like that.