What you are training
Guitars take up more of the spectrum than any other instrument in a band, which is why they cause the most masking. Acoustic body lives around 150 to 250 Hz, boxiness near 400 Hz, pick attack and string detail around 2 to 4 kHz, and fizz above 6 kHz on an overdriven electric. A guitar that sounds great alone and wrong in the mix is almost always taking low-mid space that belongs to the vocal or the bass. This drill applies an EQ move to an acoustic or electric part and asks you to name the band, so the next time a guitar fights the vocal you can hear the collision instead of guessing at it. Eight rounds cover the full range; the boxy and fizz rounds are where most people discover a blind spot they did not know they had.
After this drill, the plan: The daily plan drills guitar low mids first, because the 250 to 500 Hz cut is the single move that fixes the most guitar-versus-vocal problems.