Aileron differential is deflects the downward moving aileron less to help alleviate the adverse yaw caused by the increased lift of that wing. It will only work either right side up OR upside down due to the aileron orientation in the given flight attitude. If you dial in aileron differential for flying straight and level right side up as soon as you roll inverted the down aileron just became the up aileron and you have increased the yaw.

I hope that helps.

BTW - I personally don't use differential on any of my planes.