Logic: Side chain compression (ducking)

For this tutorial you will need a control signal and an affected signal.
For our side chain example we will use a kick drum (control signal) to trigger a compressor which will dynamically attenuate (or duck) a track with a sustained note (the affected signal).
This means every time the kick drum plays the sustained note will be compressed.

To accomplish this follow these steps:

1. Create 2 software instrument tracks (You can use audio or instrument tracks for this effect, but for our example we are using instrument tracks)
2. On the first track insert a software instrument with a drum kit and record a kick drum on every quarter note for 2 bars. After your done, insert Bus 1 into the sends field of the tracks channel strip and turn it up.This will automatically create an aux track with the input as Bus 1, this is unnecessary for what we are trying to do so go to the aux track channel strip and change the input to no input
3. On the second software instrument track record a sustained note that plays for 2 bars ( you can do this by opening up the ES1 on a software instrument track and using the default patch).
4. Now on the track you just recorded the sustained note on ,add a compressor plug in to it's respective channel strip. In the side chain field of the compressor click, hold and select Bus 1 as shown in the image below.

1. Push play on the transport
2. Turn the compressors ratio all the way up and begin to pull the threshold down. You should hear the sustain note begin to fluctuate in volume in accordance with the kick drum hitting, if not try pulling up the gain on the compressor 

Ducking is an effect commonly used in radio and pop music, especially dance music. It is an effect where the level of one signal is reduced by the presence of another signal, through the use of side chain compression. A typical application is to automatically lower the level of a musical background track when a voice-over starts, and to automatically bring the level up again when the voice-over stops (in Movies and on radio-broadcasts).

From WikiAudio

Side-chaining uses the signal level of another input or an equalized version of the original input to control the compression level of the original signal. For sidechains that key off of external inputs, when the external signal is stronger, the compressor acts more strongly to reduce output gain. This is used by disc jockeys to lower the music volume automatically when speaking; in this example, the DJ's microphone signal is converted to line level signal and routed to a stereo compressor's sidechain input. The music level is routed through the stereo compressor so that whenever the DJ speaks, the compressor reduces the volume of the music, a process called ducking. The sidechain of a compressor that has EQ controls can be used to reduce the volume of signals that have a strong spectral content within the frequency range of interest. Such a compressor can be used as a de-esser, reducing the level of annoying vocal sibilance in the range of 6-9 kHz. A frequency-specific compressor can be assembled from a standard compressor and an equalizer by feeding a 6-9 kHz-boosted copy of the original signal into the side-chain input of the compressor. A de-esser helps reduce high frequencies that tend to overdrive preemphasized media (such as phonograph records and FM radio). Another use of the side-chain in music production serves to maintain a loud bass track, while still keeping the bass out of the way of the drum when the drum hits.
A stereo compressor without a sidechain can be used as a mono compressor with a sidechain. The key or sidechain signal is sent to the first (main) input of the stereo compressor while the signal that is to be compressed is routed into and out of the second channel of the compressor.
From WikiAudio