The pump: how one track triggers another
Sidechain compression allows one audio signal (the sidechain input) to trigger a compressor on another signal (the main audio path). The classic use case is EDM: route the kick drum to the sidechain of a bass compressor. Each time the kick hits, it triggers compression on the bass, ducking the bass volume in sync with the kick transient. The effect is a rhythmic pumping or sidechain wobble that glues the kick and bass together and creates that distinctive EDM dance sound.
This is distinct from using the audio being compressed as the detector. A normal compressor listens to its own input and compresses based on its own peaks. A sidechain compressor listens to a different input and compresses based on something else's peaks. This allows the compressor to be triggered by rhythm (a kick), frequency (a filtered version of itself), or another track entirely.
Beyond the kick: sidechain uses in mixing
In mixing, sidechain compression has many roles beyond the iconic EDM pump. A vocal sidechain compressor (triggered by a click track or the rhythm of the song) can add rhythmic movement. A bass sidechain triggered by the kick is standard in hip-hop and pop production. A synth sidechain triggered by a filtered kick (only the low frequencies trigger the compressor) can create a duck that's less extreme and musically transparent.
The parameters matter: a fast attack makes the pump tight and audible; a slow attack lets the sidechain signal attack before compression kicks in. A medium release (100-500ms) creates the pumping feel; a very fast release (10-50ms) creates a tighter, less obvious effect. Most producers use sidechain compression deliberately for character, but it can also solve problems like bass and kick masking when subtly applied.