The down moving aileron makes more lift. More lift is more drag. More drag pulls that side back.

The up moving aileron decreases the lift on that side. Less lift is less drag. That side moves forward while the other is pulling back.

Rolling into a turn is more efficient. Yet, as you can see, those ailerons yaw the plane the other way. Not good.

The longer the wing, the more leverage the ailerons have. So sail planes usually have problems that go unnoticed in the average power model, like Ugly Stiks and such. Any wing with dihedral winds up fighting itself. Another reason gliders or floaters benefit from differential.