Don't expect a smooth, calm, uncomplicated affair, because with this aspect between you, it's anything but! You challenge your lover on the deepest levels and you draw out all their darkest emotions. You challenge them to face their past, their roots, the forces that they came from that were beyond their control but which shaped them and made them the person that they are today.
Needless to say, this can be an uncomfortable, difficult, even painful process for your partner! They might discover things about their past that they just don't want to think about, much less deal with. But this aspect can have a really cathartic effect, because it forces transformation. The person they once were, operating in the world based on a series of influences, experiences and encoded lessons, doesn't necessarily have a whole lot to do with the person they become once they work through all those old, dark feelings. You help them to understand that truth, and to go through the transformation that needs to happen. They become a brand-new person as the result of this relationship, and that's a good thing -- even if it doesn't feel like it in the moment. They should take this as an opportunity to peel away at the layers of their subconscious mind like an onion, and they come through it stronger, more self-aware and emotionally empowered.