What you are training
Every live mix has a ceiling: the level at which the vocal mic starts to feed back through the wedges or the mains. Engineers who understand gain before feedback push toward that ceiling with control; engineers who do not either run the vocal too quiet or chase rings all night. The ceiling is not one knob. It is mic choice and placement, wedge angle, stage volume, how many mics are open, and where the EQ has been cut. This drill puts you on a virtual stage with a vocal in a wedge, lets you raise the gain, and asks you to stop at the last stable point before the ring. It shows how much headroom you left on the table, or how far past the edge you went. Six rounds teach the feel of a system near its limit, which is the feel of every real gig.
After this drill, the plan: The daily plan pairs this with ringing out a wedge, so the ceiling you learn to feel becomes one you know how to raise.