I never go into a full explanation of it, but basically it is different entities that are somehow linked together, but basically exist on different planes, and in fact are quite different, but the idea is that the combination of each entity with its opposite is the creation of the philosophers stone, and that they “switch places” so basically one becomes the next one. So in some cases it is as if one “kills” the other one because in some cases life and death are reversed.
Astaroth could give you a better explanation with personally relevant examples, just skip the part where she tries to talk you into possession, stepping outside the circle, or whatever kind of temptation is offered.