Dynamics processing, such as compression, limiting, gating, and expansion, can sometimes be keyed or triggered by an external audio source – usually another track or mixer channel. An example of this is gating bass guitar and kick drum tracks. By keying a gate inserted on the bass track from the kick track, the timing of the bass can be tightly synchronized with the kick drum.
The track that’s triggering the effect is routed to an input known as a key or sidechain input on the processor. The key or sidechain signal passes through a circuit in the processor known as a detector, which senses the audio level and sends control signals to the gain cell to tell the device how to react to the signal.
In some cases, you want to use the same signal you are compressing as the key input signal. An example of this would be de-essing. A compressor with a key/sidechain input can accomplish de-essing by Y’ing the input signal to the compressor, routing one side through the compressor as normal, and running the other side through an EQ and boosting the offending frequencies, then running it into the key input. The compressor reacts to the boosted signals, and compresses them, helping control sibilance.