Side-chain compression for ducking background music under a vocal track
Side-chain compression uses the voice track to trigger the compression placed on the music. The compressor clamps down and reduces the volume of the music when the voice starts to speak. This allows you to "duck" the music below the voice and then have the music return to normal volume.
This is very common in Radio and TV and can also be used for other applications wherever you want some BGM to drop below a voice track.
