Side chaining

A side chained sound effect is one that changes one signal depending on another signal.

Side chaining is often used with compressors. Usually, a compressor will react to the amplitude of a signal and change that same signal. Only one signal is used by that effect and that same signal is the one that changes. A side chained compressor takes two signals. It compresses one of the signals when depending on the amplitude of the other one.

Ducking

Side chained compressors are often used to hide a signal under another. This is called "ducking". In electronic dance music, for example, the bass is compressed with the hits of the drums, so that the bass "disappears" during the drum hits. The bass is the signal that the compressor changes, but the compressor reacts to the amplitude of the drums. With vocal reverbs, the reverb is compressed whenever the vocals come in, so that the reverb does not smear the vocals and is heard only when the vocals subside.

Dynamic equalization

A dynamic equalizer can also be implemented with a side chain. Perhaps, an equalizer is meant to affect only portions of a vocal track where an offending frequency appears and not the whole vocal track. That will preserve the qualities of the vocals in most of the track. That equalizer could be designed to act only with the amplitude of a set of frequencies extracted from the signal. Technically, such a dynamic equalizer is a side chained effect, as it impacts one signal based on another. In practice, the second signal is just a subset of the first signal (a frequency or a set of frequencies extracted from the signal). It may not be seen as a separate signal and may be computed by the equalizer itself.