What you are training
Bass and acoustic guitar usually arrive as two signals: a DI with the clean fundamental and a mic or amp with the character. Blended right, you get the weight of one and the tone of the other. Blended without checking polarity and time alignment, the two cancel in the low end and the instrument sounds thin no matter how much you boost it, which is the most common invisible problem in a session and on a stage. This drill gives you a DI and a mic of the same performance and asks you to blend them: find the polarity, align the arrival, and set the ratio. The check is to sum to mono and flip polarity until it sounds worst, then flip back; the drill makes that reflex. Five rounds cover bass, acoustic guitar, and a kick drum inside-and-outside pair.
After this drill, the plan: The daily plan drills polarity and alignment as a fixed two-step routine on every multi-source input, before any EQ.