The low pass filter is controlled by a rotary knob and is great for reducing any sort of high end hiss. You basically just roll back the threshold knob till all or most of the high frequency noise disappears in the quiet or dull bits, being careful that the setting used does not reduce the high end content of the overall sound when the full force of music kicks in.
The downward expander can be used to cut out all unwanted noise when the music isn't playing. It shuts down the whole signal when it falls below a level set by the threshold knob, there are three expansion ratios available and three levels of attenuation so there is plenty of versatility and the attack time is program dependent and very fast so you don't lose any transients, the release time is set by a rotary knob.
I've had one of these for around twelve years and its invaluable for getting rid of tape hiss, the noise from effects returns and sound modules, and the hum and hiss from guitar amps. You can stick it across your mixing desks outputs when doing a final mix and it will 'top and tail' it without cutting or glitching the start of the song and, with careful setting up and dependent on the type of player/playing, you can use it unobtrusively while recording guitar to cut out all the amp grunge in the silent bits.