I think they were talking about mapping elevator trim. You simply need 2 buttons, one for "trim up" and one for "trim down".
A spare hat is good for this type of thing.
You basically fly with combat trim off. Adjust trim up or down to keep steady flight. This helps when you change speeds (like in a dive or a zoom or something) but generally isn't 100% vital. Unless you talk to P38 pilots, a lot say it's pretty important for that particular plane.
How to get it back? You are simply changing the trim. You have too much nose up, trim it down again. Either just manually re-trim it, or hit CTRL-X (toggle key for Combat Trim). If you toggle combat trim on then off, your trim will stay at that point until you manually change it again. If you leave it on, well that's another question.