@Crumbly WriterI'm highly interested in my character's back story, but didn't want more than a page or two of exposition on how we get to where we are. It's much better if it comes out over the course of the story.
Think about it from a first-person narrator perspective: which is more likely?
"I did this, and this,and this and that and the other thing happened, and this went on for years, then a truck hit me, then all these things happened."
or
"Damn! A truck hit me and I didn't die, I wound up in the past!"
The biggest single event in their telling their story is the triggering event for the do-over. However, it doesn't mean their backstory isn't interesting, it's just "oh, and I'm all screwed up by years of this, that, and the other thing" is bad writing - that should come out in the story, and most likely they don't actually realize how screwed up they are going in.
I probably do too much introspective exposition in the early chapters and pull out some of those things, but, to me at least, the more you meet the MC naturally the better."
That said, and referencing the lightning strike, 'Lightning In A Bottle' does start with a strong, relevant opening. I wouldn't say it's exactly backstory, but it's very important to the plot. That said, the 'rules' for that do-over make it relevant; one that's more open-ended would be hard-pressed to end up anywhere even vaguely similar.
My MC wouldn't be caught dead (pun intended) in a car anywhere near where and when his fatal meeting with a truck occurs, not unless the whole do-over is a colossal failure and he wants a third try, anyway (no, that's not a spoiler!).