From homeboys webpage: about those trim wheels,
Those Trim Wheels, They Ain't
Those two dials near the gimbals of a CH stick are referred to in CH Products literature as "trim wheels." That's an unfortunate term because us aviation types can't help but think that means "aircraft trim" and we are tempted to try and use those wheels to trim our airplane. A better choice of term might be "pot trim wheels" or something like that. You see, this wheel is physically attached to the axis pot and can be used to center the pot (or equalize the travel of the pot to one side and the other). The problem with trying to use them for aircraft trim is once you've calibrated your stick and then you move one of these trim wheels, you throw off your calibration and it is very difficult to get the pot exactly where it was when you did the calibration. The precision of these wheels is not that great and just the slightest movement can throw the pot off quite a bit. It is my opinion that you carefully lock the trim wheels in their detent and never touch them again.
In truth, these trim wheels are a bit of a hold-over from the Gameport days where centering the pot was a legitimate issue. Because of the way USB devices are designed, this is not that much of an issue anymore. It is very rare that these USB sticks need any sort of pot trimming.