A few years ago I was logging a lot of hours in flight simulators. I had the idea that I would one day get my pilots license. That has been on hold for a while, but I learned one lesson that is useful in audio. Small aircraft (an likely large ones, too) have a trim control. The trim control allows the aircraft to fly level (or climb or descend at a fixed rate) while the stick is in a neutral position. It essentially adjusts what "neutral" is so the plane will do what you want without your constant intervention. The goal of the trim control is to reduce the pilot's workload so he can focus on other tasks. I'm greatly simplifying here for the sake of illustration, so go easy on the comments, pilots...
When it comes to mixing audio, we have a pretty high workload. We might have 20, 30, 40 or more channels to keep an eye and ear on. Inside each channel are dozens of parameters; input gain, EQ, comps, gates, aux sends, panning, level. There is a lot to do. Thankfully, as we are now almost fully in the world of digital consoles, controls exist to help lower our workload so we can focus on other things. One of them is the trim control.
Different console manufacturers treat this word differently so let me define a few terms. Gain, at least in this post, refers to the mic (or line) pre-amp stage; the initial gain of the channel. Trim is a digital control after gain that will add or subtract level. Some companies call "gain" " trim," and that's confusing
I like to mix with my faders at or near unity. I also like to hit my preamps pretty hard so as to maximize signal to noise ratios and A/D conversion. Sometimes those two goals are at odds. For example, if I turn my hi hat channel's gain up enough to really light up the preamp, it will take your head off in the PA. I could turn the fader way down, but then I'm not at unity, and I don't know where it's supposed to go back to if it gets moved inadvertently. Enter trim.
When I do sound check, I'll crank up the gain on each channel until I'm getting a good level on my preamp. I set each fader at unity, and take a listen. If there needs to be more level for that channel in the house, I'll add some gain with trim. If I need less, I'll dial back the trim. By the time I'm done with sound check, I have a decent starting mix with all my faders at unity. From there, it's slight adjustments to suit the mood of the song.
With my faders at unity, I have the greatest resolution for making subtle changes in the mix, and I know where to reset them after a song change (well, now I actually use snapshots, but that's another post). If you've not used the trim control before, give it a shot and let me know if it helps.