exponential is a mathematical curve were the control gets steeper away from center,using Futaba you can get more or less response around the center position by dialing in negative [-] or positive [+].To soften or slow the servo response to the linear stick movement around the center point,dial in - %,t o speed the movement dial in + %.The % change is in relation to the linear movement of the stick around the center position,usually the expo on the throttle works at the ends instead of the center.All expo works basically the same ,some brands use positive to soften the center,some at positive speed the center. Hope this helps.