You can use a signal from any other track as a sidechain (lowcut/hicut/threshold/detection between peak+RMS/hysteresis!!!!!!) and then you have 4 filters (+/-12dB) (30-300Hz, 100-1999Hz, 400-20 000Hz, 2 000-20 000Hz with a Q from 0.3-4) with added attack/hold/release. Going out you can adjust the master gain.
Want to shave off cymbals on the tail of a tom track and JUST on the tail, while keeping the attack sharp and cutting through the mix? Want to tame grit on the electric guitar madness every time the vocals come in and have it come back when the singer keeps it's mouth shut? Got a subtone you want to rattle the whole mix and get all the other signals lowend out of the way for a few seconds without drawing automation? Want an envelope filter on a track controlled by a click, the snare, the cowbell, the tibetan bone flute or whatever? Highly recommended. As if that weren't enough, you can have a volume envelope as well, on top of your eq - make it louder every time the sidechain says, duck it... phew!!!
Audio units and VST - a free plugin from Heel Audio. It's fantastic!