Grimace Musically

noun
1. An ugly, twisted expression on one's face typically expressing disgust, pain or amusement while playing an instrument or vocalizing
2. One of the requirements for recruitment into a heavy metal rock band

"Tomorrow: Open audition for high-profit heavy metal rock band. Need to know three chords and be able to grimace musically"

"...and what the dickens is "grimace musically"?"