ORIGINAL: Ed_Moorman
Sub trim is misnamed. It sould be called "Center Adjust," in my opinion. Your computer radio has end point adjust and center adjust, only they were called "travel volume," and other names, and "sub trim" by orientals with very little knowledge of the English language. The name sub trim has messed up people new to computer radios for 20 years.
As has been mentioned, it has nothing to do with your trim levers and it is misused a lot.
When you program a new airplane into your radio, the first thing you do without fail is RESET. Clear out the old airplane settings. Do this even for a new radio. It may have been used to demo at your local hobby shop. (And don't tell me this can't happen.) Next, put on the servo arms. If they are not square, that is 90 degrees to the axis of the servo, use sub trim to center the arm. Don't touch your sub trim again.
best answer yet!
AFAIK the only manufacturer to get this right was ace with their micropro8000. in one simple function you set up each end point and center. and because this was done by specifying the actual pulse width of each point you weren't limited to a few % of movement either. so in one simple step you could set up direction, differential and overall travel. you could even set the "center" and one end point to the same value and therefore make the servo respond only when the stick is moved in one direction and not the other.
dave