A "hook" is something to keep the reader reading. The most common and best known hook is the reader wanting to know what will happen. Does the main character live? Who is the murderer?
The second type of hook, which I call mystery, is wanting to know what is happening in what the reader is already reading. For example:
Yes.
She was there again.
Watching him, his every move.
(Fade to Black, Graham)
We don't know who "she" is, that's critical information, and it would've cost almost nothing to mention who she is. So, information is being deliberately withheld.
You might be annoyed, but most readers will want to keep reading to find out who "she" is. And that was the author's goal. It's a hook.
We also don't know why she's there or where she's at, but those are more normally not told right away because there's more important things to do.
Continuing:
She wanted something, of course. Or he was due for a lecture, a long litany on how to live his life.
He'd been splitting logs outside his cabin when he'd first become aware that she was there; he continued to chop firewood. If she was going to haunt him because she wanted something...
(Fade to Black, Graham)
We still do not know who she is, though the last line finally tosses out a hint.
A reader might not like the sound of being "hooked". (You can think of yourself as a fish or someone a drug dealer hopes becomes an addicted buyer.) But it's always nice to have a motivation to keep reading, and the hook can involve the reader in the book, which could be good for the reader (especially if the book is an escape).
The Problem of Not Knowing Context
Suppose a character decides to stay in bed all mornng. That says something about the character if we know the situation and why the character is deciding to stay in bed.
If the character normally wakes up quickly and now is not, or if the character is normally responsible but now is being irresponsible, that tells us about how the character is feeling at that moment. But that requires knowing the character's normal behavior.
Having mystery means the reader doesn't know important context for whatever is happening. That creates difficulties understanding the character and the story. For example:
Friends give one type of lecture, professors another. Father's lectures tend to be different from mother's. Yes, the reader finds out in a few lines that it's his mother. The reader could stop, try to remember that line about lel\ctures, and re-interpret it properly. But we know that's not going to happen, and a reader shouldn't have to do that.
Without context the reader also can't know the implications of an event. There's no cell phone coverage? That could be trivial. Or annoying, or dangerous. But there's no way to know without context.
So. We have already met this principle:
This principle limits how much a start can be meaningful. It also limits the value of mystery. I don't like prolonged mystery – I read my stories for meaning, and the mystery deliberately takes that away. You can decide you like deliberate mystery.
That's an obviously intentional mystery. This is not leaking any information about her as a character. Page 8 is the flashback to what happened, which in my opinion would have made a much better start.
Actually, she just snapped, because "Snapping is easy when you're already brittle from the worst possible thing happening." More mystery. But the reader was warned by the start.
But now let's talk about unavoidable mystery in the start.