Honestly, I've always found the emotional framework in those stories more interesting than the physical act itself, which is kind of weird to admit. The best ones don't just focus on the predator's glee or the prey's terror—they get into this weirdly intimate psychological space. It's about total, irreversible submission for the prey, which can be framed as horrifying or strangely ecstatic, and for the hippo, it's this overwhelming, possessive affection that's literally consuming. They'll use sensory details like the warm, humid darkness of the mouth, the rhythmic pulsing of the throat, to anchor those big feelings in a physical reality.
I read one where the human character actually found peace in being swallowed, like all their anxieties were being dissolved in stomach acid—it was a metaphor for letting go of control, but taken to a literal, monstrous extreme. The tension comes from whether the emotion is mutual; is this a cruel act of hunger or a twisted form of love? That ambiguity is where the real spice is for a lot of readers. The writing can get surprisingly tender, focusing on the comfort of being surrounded and held, even if that 'holding' is digestive.