What you are training
Mid/side processing lets you treat what is in the centre of a mix separately from what is at the edges, which is powerful and dangerous in equal measure. Widen the sides too far and the mix hollows out in mono, which is how a large share of people will hear it. Narrow them too much and a stereo record turns into a mono one with a bit of decoration. The ear can learn the width of a mix the way it learns a frequency band, and that is what this drill trains. It plays a section at a hidden width setting and asks you to judge it, then shows what happens to the same setting when the mix collapses to mono. Six rounds teach the two questions every width decision has to answer: does it sound bigger, and does it still work on one speaker.
After this drill, the plan: The daily plan pairs width with a mono check on every decision, because the decision is only good if it survives the collapse.