For Hover Expo can be used in place of or in addition to dual rates to reduce how much stick movement is turned into actual servo movement. I personally prefer expo over dual rates. POSITIVE expo will increase the sensitivity around center. Meaning you'll get a lot of movement on the servo for even a little on the stick, and it exponentially tappers off at the rate you set. Negative Expo decreases the sensitivity, meaning you need big control stick inputs to make the servo's move even a little. (Hence why it's good for hovering) Some radio's treat expo back-wards so make sure you test it well before you fly it. The nice thing about expo is it tapers based on the amount of expo you apply, so you can tweak it just a pinch if you need just a bit more control, or want a bit more aggressiveness.