every modern car has been fed with some values of petrol and air mixture to make it run efficiently. short trim means how much fuel is additionally added or subtracted from that factory fed value. positive trim means more fuel is being fed and negative means fuel is being lowered than factory fed values. long trim is more of an avg of many short trims.
positive trim means more air is entering the combustion chamber and ECU sprays more fuel to run the car better. ( fuel rich condition) negative trim means more fuel was going in combustion chamber in comparison to air and ECU reduces the amount of fuel ( lean fuel condition)
the total trim should remain between positive or negative 10. ideally it should remain close to zero but that happens rarely. + - 3 is OK. anything above needs attention.
