Sidechain Compression

Sidechain compression is when a signal, other than the main input, is used to control the amount of compression. A classic example of this is a kick drum being used to duck the level of a bass line. The compressor is inserted on the bass track but the kick drum is used to trigger off the compressor, which in turn momentarily ducks the bass signal, creating a pumping effect.

Tutorial on how to use the SC3 LADSPA plugin for side chain compression

Tutorial on Sidechaining in Qtractor using Ducka