What you are training
Compression is the hardest process to hear and the easiest to overdo. Its sound is not a tone, it is a change in how the loud and quiet parts relate: a snare that stops jumping out, a vocal that stays put, a guitar that loses its pick attack. Until you can hear gain reduction happening, every compressor setting is a guess and the usual result is a squashed, lifeless track. This drill plays a phrase twice, once dry and once with a hidden amount of compression, and asks which is which and roughly how much reduction was applied. You are listening for the transient, the sustain, and the breathing between notes. After eight rounds you will know whether you can hear 3 dB of reduction, which is the amount most real mix decisions are made in.
After this drill, the plan: The daily plan moves from detecting compression to setting it: attack and release first, ratio last, always at matched level.