Whenever a VST version of the plug-in is used you need to set up the multi channel track before inserting the PSP MasterComp, this is a usual procedure for VST 2.4 plug-ins.
Side chain filters attenuate or boost low or high frequencies depending on the setup. Side chain filters allow you to choose how the compressor should react depending on the frequency. Sometimes you need to have a heavy kick or bass pumping so you boost low frequencies in the s.c. In most cases you need the compressor to be lighter on low frequencies as in most cases the kick still has enough energy in the mid range for proper and musical compression so you cut lows.
Side chain is a control signal path, whatever you do on a side chain signal reflects in the processor's behavior but doesn't change the processed sound directly.
Regards,
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Mateusz Wozniak
PSPaudioware.com