This is a post by mastering engineer Barry Gardner who operates SAS Mastering
What is side chain processing?
Side chain compression is often seen as a complicated production technique but assuming you understand the basics of compression there should not be any troubles understanding how it works. Most audio compressors work by controlling the input signal using the same signals dynamics, so for example when there is a loud section in a vocal recording the peak in the vocal gets reduced in volume. This has the effect of evening out the level of the singing. It is a very common and useful technique for many types of instrument and most modern music mixes will have a reasonable amount of compression occurring.