Heh, on JR radios, "negative" expo makes things WORST! We use positive Expo to soften control around center.

Basically the servo is trying to move a pushrod via a circular link, in a straight line.

This means that there is the most movement around TX stick center, as the circle described by the servo arm moves the most in a linear direction as the servo is close to center.

Expo acts to even things out.

I've seen many people really overdo it with expo. IMHO over 50% causes you MORE problems, because once you are past a certain point on the stick, movement accelerates.

Try this on a rudder and you'll find that you'll be unable to keep your plane running down the centerline, as you'll always be overcontrolling.