A sidechain compressor acts like a normal (wideband) compressor but has the ability to filter the detected signal before sendig it to the compression stage.
Normally a compressor uses the full range signal to detect a level above the threshold. A sidechain compressor provides a filter (or equalizer) for the detection signal to reduce the bandwidth or to remove some frequencies from it. For example: If you cut all lower frequencies from your detection signal the compressor will decrease the volume of your track only if some loud highs appear. With this technique you are able to reduce the consonants of a vocal track, remove "rumbling" of single notes from a bass guitar or level the dynamics between an clean and a distorted guitar on a single track.
Calf Sidechain Compressor provides two independent filters to give complete control over the sidechain signal. You can listen to the filtered signal through the "S/C Listen" button to set the filters values exactly. The filters' LED's display the status of them. A blue light shows a bell or shelving filter, red light indicates a high-, low- or bandpass.
For deessing or derumbling you can choose a "split" mode from the list, too. While in this mode only one filter is set to active. Choose a split frequency with this one. The other filter is used internally to split the signal to be compressed at the same frequency as the sidechain filter. After compressing the chosen frequency range through the (also filtered) sidechain it is added to the untouched frequency range again. Split mode gives you the ability to reduce a defined frequency range without affecting other frequencies. It can be useful for example to reduce an outstanding crash cymbal on a bassdrum in a mixdown without affecting the pressure of the kick. Or perhaps to level sharp consonants on a recording of a choir. It also may add some missing "air" in a vocal recording, a stings track or what ever without raising attacks or loud enough heights.
A specialized version of the sidechain compressor is the deesser. It works mostly like a sidechain compressor but with specialized settings to be more effective on vocal tracks or high frequency compression.
If you have a signal containing complex or wideband frequency ranges and have problems in different ranges then a multiband compressor could be the right choice.
For more information about compression and the essential values please refer to the compressor's manual.