Easiest way to understand expo is to try it. Set up a switch to turn on 100% expo on the rudder, then turn the switch off. Move the stick 1/4 of the way left or right, and flip the switch on/off. You'll notice how with expo on, the rudder doesn't move nearly as far at 1/4 stick. Basically, it allows you to make the controlls less sensitive near neutral. Pushing the stick all the way to either side will still give full throws no matter what you do with expo. Make sure while controlling the plane that you use small movements. I've got a super cub so I understand a bit about what you're going through. The original controls were mushy and slow and you probably got used to moving the sticks fully in various directions as you flew. With better electronics, you'll have to get used to smaller stick movements.