Backstory is like an onion: you can peel it one layer at a time

I'm currently reading The Broken Kingdoms by N.K. Jemisin.  It's the second book in a wonderful fantasy trilogy, and there are many great things to say about it, but I'll focus on one.

It has a remarkable narrative structure, where it's as though the main character is telling me a story, and through this, she's able to give seamless flashbacks and exposition.  She can go on tangents to explain back story, to explain why this event is culturally significant, to explain why it's personally important to the main character.  I have no problem with flashbacks, but having the subheading of <ten years earlier> always strikes me as less than ideal.  Sometimes you gotta do what you gotta do for expediency's sake or because you're investing your efforts elsewhere, but the way she's worked around it here is impressive and worth a look.

The thing that's really neat about it is that she manages to do this while still maintaining suspense.  Things are kept from the reader.  The narrator touches on issues, then turns away from them.  Now, this usually irritates me to no end.  If I don't know something only because the book refuses to tell me, that's irritating.  But here, I'm never irritated, which is impressive given that at one point in the first book of the series, the narrator started half a sentence of a flashback, then changed her mind.  "My mother - No. Not yet."

And I was completely okay with it.

How?   What does Jemisin do to make this work so well?  I have theories.

• Flashbacks and explanations are kept short.  They're reminiscent of fleeting memories.  They are always related to the moment, rather than for something that will turn up at a later time (or not solely for that), so it feels natural for them to be discussed.
• Similar to this, flashbacks and legends are used regularly throughout the book, so this is a device we grow accustomed to.
• It's written in the first person, so we get to know the narrator, not necessarily to trust them, but to trust their story telling.  Part of this is that the narrator is a full, flushed out character.  She has flaws and strength and moods and reasonable reactions.  There's a sense of authenticity to her.  I trust that she's not actively keeping things from me out of spite or just to be obtuse, because I know the character wouldn't do that to me.  So this narrative device is completely bound up in characterization, the very foundation of the book.
• Closely related to that, the reader gets to know the character the way they get to know a person in real life: slowly, pieces at a time.  The main character opens up little by little, revealing more and more.  So, just as a person you just met wouldn't tell you the graphic details of their father's death, the narrator doesn't mention it in the first pages of the book.  Through this, the reader grows more and more invested in the character.
• Closely related to that, in a way, the reader doesn't want to push at issues that are too personal.  When information is withheld from me, I can detect that it's painful for the narrator to talk about it.  This is because her reactions, the way she talks about (or talks around) painful things is the way I or people I know talk about painful things.  When she glosses over things or changes the subject, I respect that.