Logic's Compressor plug-in is being used for 'ducking' in this screenshot. The compressor is inserted on the first track (a backing submix) but its actions are controlled by the second track (vocal). When the vocal signal exceeds the threshold, the submix is automatically compressed and reduced in volume. The submix itself has no impact on the operation of the compressor. Note that, to compensate for the attack time of the compressor, the vocal track has been slightly advanced ahead of the submix.
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