Sidechaning has been greatly improved in FL Studio 9 over the previous versions. If you want to hear one of the best examples of sidechaning in action, just listen to some of the music by deadmau5. The easiest way to explain sidechaning is to actually hear it, however if you require an explanation, it is basically used to 'duck' certain sounds (bass-lines for example) in order to have other sounds more defined (kick for example).
This tutorial will show you how to sidechain using FL Studio.
