Problems with Emotional Impact
There are two ways that an event can lose emotional impact by being relegated to the past. The first is spoiling.
We have already discussed how telling about the future, for example a single important fact in a goofy first line, can spoil the emotional impact of that event when it naturally occurs in the regular timeline.
This is the same principle: When a story jumps back in time, essentially the events that have occurred so far in the story are all potential spoilers for the events in the flashback.
For example, When my character backflashed on the story of how she damaged her leg, the reader knew that was going to happen, so the emotional impact was blunted. (I didn't even try for emotional impact.)
One book by Deb Caletti begins in the middle of the chronological timeline with the main character's husband going missing. But interleaved with this search-for-him story is the story of their original affair, leading to their marriage. For me, knowing that they got married half-spoiled the story of their affair.
For me, anything in the past is less emotional. In one book, instead of reading about her baby vomiting on her, we read her blog a few minutes later about it. The emotional impact was nearly lost (for me).
That makes sense in a way. I was not reading about a character having her child vomit on her, I was reading about a character blogging.
More generally, I try to put myself into a book as if it is really happening. That maximizes emotional impact. But if I am in a flashback, I must remember that it's an event in the past, or else I'll become confused about the order of events. How can I remember something is in the past and treat that event like it is really happening?
If the flashback is long enough, I'm guessing that the problem of fading is reduced, to the extent that the reader becomes asborbed in the flashback. But that still leaves the problem of spoiling, and it essentially creates the problem of time jumping.
The Unemotional Past
So, if you want a scene to have emotional impact, you should try to avoid starting your book after that scene.
If you decide to start after that scene, you can still try for emotional impact. You could get some, I predict only that it will be less. Or you could give up, and of course the scene could still be interesting.
But there is a cost in trying. There are ways that we make a scene interesting. Like creating suspense. But it's hard to create suspense when the reader has already been told what will happen.
Surprise isn't going to work well in a flashback, and neither are happy outcomes. So maybe those shouldn't be your goals.
In fact, maybe a flashback should have a completely different goal, one that exploits their advantages.