What you are training
Sibilance is the hiss on S and T sounds that a bright vocal chain turns into pain, and the de-esser is the tool that catches it. The skill is threshold and range: enough reduction that the S sits with the rest of the word, not so much that the singer sounds like they have a lisp. Most over-de-essed vocals happen because the engineer set it by looking at the meter rather than listening, and most under-de-essed ones happen because the additive EQ was too aggressive in the first place. This drill plays a vocal phrase with a hidden de-esser setting and asks you to judge it: too little, right, or too much. Six rounds train the narrow window where a de-esser is doing its job, and teach you to hear the lisp before a listener does.
After this drill, the plan: The daily plan drills the vocal chain in order, so you learn to fix sibilance at the EQ stage before the de-esser has to work hard.