This is an important technique to understand for both engineering and creative purposes. Basically, sidechaining is the use of an external signal’s dynamics to control the compression of a different track. For example, everytime a bass drum happens, it triggers the bassline to duck down a few decibels. Another time you may want to use this is if you have a lead instrument (like vocals) that is being crowded by other instruments in the same range (like guitars or keyboards). You can put a compressor on the keyboards or guitars and everytime there is some vocals, they keys/guitars get lowered so we can hear the vox better. This technique can be used to extreme for a pumping effect that is commonly found in house music. The entire song pumps to the beat of the bass drum, commonly and conveniently programmed to the quarter note.
Again, i suggest you watch this video with either decent headphones or real speakers. We’re dealing with low end here that will not translate on a laptop speaker too well.
Step by step review:
1. Put a compressor on the track that will be attenuated (lowered) by another signal.
2. Turn on the sidechain and select the aux or send that will be the source
3. Using a send or aux, send the signal that will do the attenuating to the corresponding bus.
4. Adjust the threshold of the compressor to achieve the desired amount of compression.
5. Adjust the rest of the parameters to make it sound nicey-nice.
* You may want to make the send pre-fader so you can hear the effect even if you mute the track going to the bus.
Let me know if this works for you.