The external sidechain enables the compressor to control its processing totally independent from the audio material running through it. If the SC Extern switch is active, compression will not be controlled by the signals from the regular audio inputs anymore, but by different signals which are fed into the additional sidechain input connectors.
If, for example, a duplicate of the input signal is processed with an equalizer and then fed into the sidechain input, the result will be frequency-dependent compression. Another example is to send the bass drum of a drum machine into the sidechain input in order to achieve nice groovy compression that is pumping in time with the music.
The creative options are almost infinite. Compression can be exactly on time or totally against it, which can of course be varied on the fly. Single instruments can be given more space in a mix according to its rhythm. All of a sudden, static sounds become vivid and sound really interesting!
Important: Your host software must support sidechain functionality in order to use this feature of the mpressor plugin!