Ditto pkh,

I use mine to center the control surfaces. Then on the maiden flight, I trim the model out using trim on the stick, then work THAT trim back out using the subtrim. This gives me centered trim at the stick and I can adjust it a click or two depending on the wind/whatever on that particular day.

So, once a model is trimmed out, I move all that trim into the subtrim so it "looks" centered on my transmitter.

Hope that makes sense.