What you are training
A compressor on a bus reacts to whatever is loudest, and on most mixes that is the kick drum. The result is a mix that ducks on every beat, which is fine in a club track and wrong almost everywhere else. The fix is a high-pass filter on the sidechain, so the compressor listens above the kick and stops reacting to it. Engineers who know this run buses that breathe with the music; engineers who do not spend hours wondering why the mix pumps. This drill plays a groove through a bus compressor with the sidechain filter set to a hidden frequency, or off, and asks you to hear the difference: pumping or steady, and roughly where the filter sits. Six rounds teach a control most people have never touched and every mix bus needs.
After this drill, the plan: The daily plan moves from hearing the pump to setting the filter yourself on a full mix, at matched loudness.