The xpressor 500 has a low cut filter with a selectable frequency which is located in the sidechain. This means that it will not affect the audio signal itself, but the way in which it will be compressed. The keyword here is 'frequency selective compression'.
Let's say you are processing a mix which has a very prominent kick drum. If you used the traditional approach of full range compression, you'd probably end up overcompressing the whole mix because everything is reduced too much.
The reason for this is the great lot of low end energy the kick produces, causing high amounts of gain reduction on the complete mix. And what makes it even worse is that your mix can start to pulsate in the beat of the kick – cool for some electronic music, but certainly not always welcome.
The low cut filter of the xpressor 500 reduces the influence that the low frequencies have on overall compression – a very easy way to apply the desired amount of gain reduction without the side effect of the track starting to pump.
This technique is especially useful in mastering or when complete mixes are to be compressed, but it can also be a handy tool for processing subgroups or even single signals, too.