I never did clearly explain how the corruption mechanic work did I? I'll put this up on the first post for later reference.

So Corruption works through exploiting the psychological weakness of the magical girl. Demons have the ability to create illusions, it's how they stay hidden from the public in the first place. With the corrupt power given by the demon lord they can learn about what that weakness is by touching the person. From there words are spouted to make the person go into a state of weakness. Then once weakened they become corrupted.

Again I should have stated this earlier sorry.