Sidechain compression triggers a compressor on one track using the signal from another track. The most common application: ducking the bass or synth pad every time the kick drum hits, creating a rhythmic pumping effect.
This technique is essential in electronic, dance, and Amapiano music. It ensures the kick drum punches through the mix without competing with the bass for frequency space.
Beyond the pumping effect, sidechain compression is used subtly to create space between competing instruments. A vocal might sidechain-duck a guitar by just 2-3 dB — inaudible on its own, but creating clarity in the mix.