-copy/paste from my post in the Dive Deep boards-
Basically, Sidechaining, best I can explain it, is having one object react to another. This is most commonly used with leveling, or compression to allow space for another sound of a similar frequency (or not), though it can be used for practically any variable tweak, knob or slide. The most common scenario for sidechain use is bass and kick drum sidechaining, in which the bass sidechains to the kick so that whenever the kick drum is playing the bass drops it's threshold/level, etc.
Now for anyone that uses FL, I can show you how to do this (and those that use other DAWs can hopefully get the general idea, though I don't know if it works quite the same way). I'm going to use the kick/bass scenario, using the Fruity Compressor method. (Fruity Balance works too; however it's much simpler.)
Now, make sure that your kick and bassline are assigned to their own mixer tracks already. When in the kick's track, add a Fruity Peak Controller as an effect. At this point, you really don't need to do anything else to it (but it helps to rename the Peak Controller something like "bassline"; it will come in handy in a sec).
Now go to the bassline's track and add a Fruity Compressor (if you already have one, still add a new one regardless). Now, you'll need to set the ratio to at LEAST 4.0:1 because the sidechain effect will not be truly effective if it's any lower.
Right click the THRESHOLD knob and select "Link to Controller...". Once the menu is opened, the first thing you'll need to do is select the peak controller from the drop-down box (this is why it helps a bit to have them named because after a while you could get confused). Make sure you select one of the options that has PEAK in it. LFO isn't mandatory, but if you'd like you can select PEAK + LFO. Then, in the Mapping formula box, type "1-Input" (without the apostrophes). This will negate the default function. At this point the menu should look like this:
Click Accept. If you go back into the Link to Controller menu again, you'll notice that the graph for the Mapping Formula has negated, and it should say "Compiled ok" underneath.
Technically, the sidechain is already active now, but you'll need to adjust it. Go back to the kick's track and go to the Peak Controller. The first thing you should do is unmute the output sound (bottom right). All of the important sidechain functions here are under the PEAK section. First thing you should do now is turn the decay knob all the way up (that's sidechain default, but you can adjust it to your liking aforehand). Now, the VOL knob or Peak Amount can be played with quite a bit. Increasing it's value makes the bassline "hide" under the sidechain more, and decreasing it will make it appear more under the kick. Just play with it a bit until you get that sound you're looking for. The BASE knob basically just affects where the sidechain is coming from; I personally don't mess with that one much because I haven't found a really good use for it. The Tension function can be pretty handy with making snappy sidechains or loose ones. When finished, it should look something like this (remember that all of the knobs can be changed a bit):
And that's about it, haha... if you have any questions feel free to ask.
