Sidechaining is linking an effect to an audio channel to use it as a trigger.
You can apply a compressor to a loop for example, and then sidechain it to a kick.
So that the compression will only be applied to the loop, when the kick trggers.
You can get some very nice effects with this.
And it is particularly good for keeping huge basslines from interfering with the kick drum.
Over doing the amount of compression will leed to sever pumping and ducking, that is particlarly characteristic of oompah music or "schranze" as some people call it.

As for you problems Acidtrash, I`m assuming you`ve applied the sidechainer to your kick, and have turned it on and linked it to the compressor.
Just play with the settings. For sever pumping, pull down the threshhold, turn up the ration, put the attack and decay on zero, and hey presto.
Oompah oompah oopah

for more suble effects back off the threshhold and ratio and also playing with the release can change the way it works depending on what you want it for.