What you are training
Mix bus compression is supposed to be nearly invisible: a decibel or two of gain reduction that makes the mix feel like one performance instead of a stack of tracks. The failure mode is pumping, where the release time is wrong for the tempo and the whole mix rises and falls against the beat. It is easy to miss on a loud playback and obvious on a quiet one, and it is the most common thing a mastering engineer has to send back. This drill plays a mix through a bus compressor with a hidden release setting and asks you whether it pumps and whether the release is too fast or too slow. Six rounds train you to hear a mix bus working, which is the prerequisite for setting one, and to notice pumping at the volume you actually mix at.
After this drill, the plan: The daily plan drills release time against tempo next, so your bus compressor breathes with the song instead of fighting it.