A book's "promise" is like the value proposition of a business, and it's defined by the reader's expectations from your title, subtitle, and cover design.
In useful nonfiction, the promise tends to be a concrete learning outcome or goal.
In genre fiction, the promise is typically a fulfillment of genre expectations in a sufficiently engaging way.
If your book's promise comes across as overly vague, mysterious, or lofty, then readers will tend to start reading with incorrect assumptions about what's inside the book.
But if a reader's experience starts to deviate too much from what they were expecting, they'll stop reading. (And maybe go so far as to leave a bad review.)
This is why a huge percentage of "hard scifi" features a cover image of a spaceship orbiting a planet, and why the titles of romance novels are all in the same sort of ribbon-y serif fonts. The visual cues help clarify the genreand set accurate reader expectations.
When a reader sees nonfiction with a title like "IMPACT," they have no idea what's inside.11If a nonfiction book's title is short and vague (which can sometimes be worth doing for the sake of punchiness), then it generally wants a longer, more descriptive subtitle to help compensate.
In fiction, I'm a big fan of the sub-sub-sub genre of literary RPG (also called progressive fantasy). It's an extremely geeky genre, and the covers of its most successful titles wave that banner proudly. If they didn't, then a reader might pick it up expecting "serious" fantasy like Tolkein, and would soon become annoyed.
Every genre has its fans, which means that the only wrong promise is a vague promise.
What a reader expects from your cover needs to match up with what's inside the book. And especially how it begins.