He doesn't actually do anything with the notes. It's about manipulating your voice....what sound to make. Like....you have different styles. Operatic, which is very full and tends to use a lot of vibrato, Jazzy is generally more mellow and doesn't use much vibrato, pop music is usually a bit more nasal, etc...then you have certain vowel sounds that are harder to sing when they are high, because for example "oo" is a more closed mouth than "ah", so when you go up high, "ah" is easier to sing. A vocal coach will give you tips on how to hit those higher notes, or to maybe make a slightly different vowel sound that's easier to make in a higher register. Like, one trick with singing high is to think low...it sounds crazy, but it's kind of like jumping a hurdle. If you think high, you tend to close off your throat...you tense up in anticipation, where if you think low, you can kind of just sail over it. Lifting your eyebrows can help, etc. And then there's the mood of the music to consider. You don't want to be rocking out during a sad song, and you don't want to be melancholly during an up-tempo number, and the style of voice you use changes with the mood. If you're singing something emotional, you're probably going to use a more breathy sound, where if you are singing something like....Pink's Raise Your Glass, you want an edgier sound. So a vocal coach will help you in deciding what's appropriate for each song to get the meaning of the song across and to help you use your voice in the best way possible. It's more about the presentation than it is singing the right notes. Robin has no idea whether or not we're singing the note that's written. He just knows if something sounds good or doesn't, and he helps to find a solution if something doesn't sound good. It might be to change the vowel sound, it might be to change the voice you are using, it might be to switch some voice parts around so that the sopranos are singing the tenor line and the tenors are singing another line, or to just slightly change the phrasing, etc. There's a LOT that can be done without needing to read the notes.
Like, we were doing Through the Looking Glass, which is in a REALLY bad key for me. The verses are in a register too low for me to get much volume, and the choruses go too high for me to hold back....I have to belt it to get it out. But if I'm belting, and the altos and mezzos, and men aren't singing loud enough, it's out of balance and I have no support under me. It sounds screechy and not good. But I physically CAN'T sing softer or I won't hit the note right. So then the vocal coach has to make sure that everyone else raises their volume to match mine so it stays in balance. And he has to make sure that the melody is heard above all the harmony parts. So when I have a melody that's too low for me to get the volume, he might ask the altos or tenors to double up on my part so I don't have to strain my voice trying to make myself heard above 15 other people. So he'll ask a few people to sing the melody with me, and the people singing harmony to sing a bit softer.
Robin is the singer in a rock band. He's a performer, but he's never studied music. He can sing, and he knows how to manipulate his voice, but he's never studied music or learned to read notes, and he doesn't know how to conduct. He can give cues for volume, or whatever, but none of the technical aspects of the written music. I hope that makes more sense...