dual rate is the rate your servo moves in relation to your stick movememt.

100% dual rate = full stick = full servo travel
50% dual rate = full stick = only half servo travel.

On the sim, set your dual rates to something like 30% or 40%, you will notice how your helis moves very softly. Now try again at 100%, your heli will jerk around, very strong abrupt movements. At 100%, your heli will probably move too too far in any direction when you give it input. At 50%, it's easier to make it hover dead still.

I"ve never mess with my expos, dual rates serve me well