So, both of these are head-hopping. In the first one, it goes from Jac’s pov, as there is a comment on something only Jac can know for sure. “Because she was…” is Jac’s pov. And then in the next line, we jump to Elovie’s pov as there’s a comment that only she can know for sure. “She’d known…” So it’s “head-hopped” from Jac to Elovie.instead of their, I'm jolted out of the bubble. When I read, for instance:
Jac's jaw dropped when Elovie walked through the door because she was wearing such a garish floral-print dress.
Elovie, struggled to smother a giggle. She'd known he'd hate the floral pattern she'd chosen.
That, for me, flows in third person and keeps me in my bubble..
But:
I felt my jaw drop. What on earth was she wearing? So ugly!
What a great reaction! I knew he'd hate this floral print and struggle to smother my giggle of delight.
In my opinion, that is head-hopping and doesn't work because it takes me out of my bubble and as that goes on it gets very confusing, for me, anyway.
I repeat, you can't have head-hopping in third person because that is the natural flow of the scene
The one in 1st person is just more obvious and jarring. But they’re both examples of head-hopping.
If we were in Elovie’s pov for this section of the story, to avoid head-hopping, you could change that to:
Elovie walked through the door and Jac's jaw dropped. Elovie suspected it was because she was wearing such a garish floral-print dress.
Elovie struggled to smother a giggle. She'd known he'd hate the floral pattern she'd chosen.
This is solidly in Elovie’s pov. We only know what Elovie is thinking.
Head-hopping isn’t about being kept in a bubble of believability. That is a broad and somewhat subjective subject. The term head-hopping is specifically about adhering to the pov chosen. It is not subjective. It can happen in 1st, 2nd, or 3rd pov although uncommon in 1st or 2nd as it’s more obvious. Omniscient Is the only pov where head-hopping is the convention.
Hope that helps. I’ll see if I can find some articles that further explain it.
The tricky bit, as @JohnBertel ‘s original post was asking about is when you want to deliberately change pov within a chapter. It is done, but there’s no set convention. So the trick is to do it elegantly and deliberately so that the reader isn’t jarred from head hopping but instead is aware of the pov shift.