Okay, that's very clear info there. And I understand what your saying. thanks.
So you can restore a vision, but to Nycto. Generally if you capture a heroine and do
anything with her, be it breeding, altar, or torture, her resistance will fall. If it falls enough, she becomes submissive, if you continue on, the vision brakes, and then you can go and drag the girl to the altar to repair the vision, but not to her original element, but to Nycto element. At that point the girl is fully corrupted and should she die in combat you can revive her with the altar again.
The issue is, the lower the girls resistance, and the "lower" her status is (going from disobedient/resistant -> submissive -> broken -> corrupted) the worse are her chances for higher hilichurl offspring.
Amber as an example very early on when you get her you can get Pyro Mitachurls from her. If she is mating too much, her resistance will drop (not submissive yet, so still in disobedient stage) and the Mitachurl will disappear as an option. Let her rest and recover and her resistance will rise again, therefore you can get the higher hilichurls again.
However at some point comes the moment, where you got enough Mitachurls and then, what use is Amber? Well
now you corrupt her. The best way is to just drag her to the altar and corrupt her through it. It gives the best results. Alternatively you can just chip away at her through different methods until her vision brakes and then restore it to Nycto vision. You can also use a abyssal wedge to instantly break her vision and then corrupt her to repair it immediately (in terms of abyss power this is the cheapest)
Essentially the first way is to convert the vision from pyro to nycto immediately and the second is pyro -> broken -> nycto. If you go without breaking the vision it will give you more pyro resistance on your altar which is why it's recommended. (though I guess now it doesn't matter much anymore cause you can just sacrifice any pyro unit to have it increase endlessly?)
The use in corrupted heroines is to use them in battle, or to get Nycto element units.