OP, confused yet? LOL.
I'm with Jim on this one - it depends, as much does with queries. Use whatever about the plot/story arc/voice/characters, etc... that will get the agent to turn or scroll down to the enclosed pages.
Some books really lend themselves to very tight, hook like pitches. And some require more of a complete plot pitch.
While I've seen some fantastic queries written like back cover blurbs, in reality, most books require you to show some of the twists and turns and secrets to hook an agent.
Agents are not casual readers - they do need, usually, more "spoilers" than back cover blurb, because you need to show those special shiny bits to stand out - especially in a crowded field. I'd guess the more crowded the genre, and more common the central plot arc structure, the more spoilers you need in the query.
For example, if you are querying a YA paranormal romance, you need to show what the love interest is and how the paranormal bits complicate the basic subplot. You wouldn't include those bits on the back cover blurb necessarily (because the paranormal and discovering that the love interest is an angel/werewolf/fairy/mermaid/dragon/etc... is only hinted at usually) but in a query you absolutely need to give those bits away as they *are* the entire hook.
Or if the major plot twist is that the character is hallucinating and half the secondary characters are delusions, then you wouldn't give that away on the back cover blurb for consumers, but you would need to show that in the query, or the agent won't understand the basic plot.
Or if in the novel, the reader won't find out until the last 50 pages that the main character is gay, but then the first 3/4 of the book will re-align to show a very different story...well, you don't want to give that away on the cover blurb, but you may very well *need* to give that away in the query.
So, write the query that works for *your* book. It doesn't *have* to give away the ending, but it can. It doesn't *have* to show the most exciting plot twist, but it can. It doesn't have to do much but include a clear and readable pitch of the primary plot, up to whatever point you wish (though I'd say usually you need more than the initial set up).
~suki