@Background Pony #39A2
Imo? Dramatic irony.
You the audience usually knows what's behind the tsun tsun
act, while her love interest doesn't.
So when she finally goes full dere dere
it's cathartic, like some kind of closure.
There is a trope called Defrosting The Ice Queen,
except the tsundere doesn't just give the love interest the cold shoulder, she goes out of her way to express her dislike towards the love interest, to further mask the fact that she harbors feelings towards them.
in Hentai it goes one step further.
MIND-BREAK. This is when a character (typically female) ACTUALLY HATES / Dislikes the "love interest" but he rapes her again and again until she's well…mind-broken, and finally is enslaved as his lover. This mind-break fetish is actually kinda common. Heck, we saw it in Healer Hero (except he cheated, he just brainwashed her directly, rather than making it a gradual process)