Sidechain compression can be used to create interesting relationships between your pads and channels. Mixing engineers and live-sound engineers often use it to help the kick, snare, or vocals to cut through the more harmonic, non-rhythmic instrumental sounds like pads, held guitar chords, or keyboards. Here's how to set it up:
First select the pad or track that you want to compress and put a compressor on it. Next hit the side chain button and select the channel and pad that you want to use to activate the compressor.
Adjust the parameters to taste: A high ratio, low threshold, fast attack, and medium-to-slow release make for a particularly dramatic use of the effect. It would be common for kick and snare sounds to compress long tonal sounds you might have mapped to the toms, so in the gif above we have set up sidechain compression on the center pad of channel 2 that is engaged by the kick and the snare. Read more about the compressor and its parameters here.