I've read people's responses and their tips about this edge but when I
read the help file it sounds very different in my mind. So:

1) Does it do what they say and make using a non-rep instrument work
better for rep songs and do nothing to help you if you're using rep instrument while singing rep songs?

or

2) Does it do what it sounds like in the help file and minimize
penalties for songs and increase the bonuses to the songs for the
rep you're in, so long as you're using an instrument?

or

3) Because #2 is true, it also does number #1.

or

4) Something I didn't name.

Iirc it just added 5 levels to my songs occasionally (not always) when using an instrument. This may be only part of the truth, though, since I didn't test it out all that much.