The side-chain input is the split-off signal used by a plug-in's detector to trigger dynamics processing, and is generally drawn internally from the input signal. However, some plug-ins let you switch between internal and external side-chain processing.
With external side-chain processing, a plug-in's detector is triggered by an external signal (such as a separate reference track or audio source) known as the
key input.
A typical use for external side-chain processing is to control the dynamics of one audio signal using the dynamics of another signal (the key input). For example, a kick drum track could be used to trigger gating of a bass track to tighten it up, or a rhythm guitar track could be used to gate a keyboard pad.
Some plug-ins feature key high-pass and low-pass filters. These controls let you define a specific frequency range in the side-chain signal with which to trigger the plug-in effect. A common production technique is to use these controls to filter a drum track so that only specific high frequency sounds (such as a hi-hat) or low frequency ones (such a tom or a kick) trigger the effect.