Marc Daniel Nelson demonstrates a quick sidechain setup to solve one of hip-hop mixing's most common low-end problems: the 808 and kick occupying the same frequency space and masking each other.
He places a Bomb Factory compressor on the 808, keyed to the kick bus, so the 808 ducks slightly every time the kick hits. The result is that each element gets its own moment in the low end without losing the overall size of either.
The before/after comparison makes the difference easy to hear. With the sidechain off, the low end is dense and the kick's attack gets buried. With it on, the kick punches through cleanly and the 808 fills the space around it.