It's probably helpful to look at two extremes.
1) Give everything away up front. Tell the story chronologically. This makes it a classic origin story: tragedy makes the (anti)-hero. Goes to get revenge/redemption. Anything from the original telling of most superhero myths (Batman, Spider-Man) to revenge exploitation films like Death Wish follow this structure.
This approach makes the story structure very straightforward, so the events themselves had better be interesting. On the other hand, if you have an interesting story, you should tell it in the most straightforward way.
2) Make the backstory the central mystery. Inception is a clear example. Cob is a man with a traumatic history, but we don't get all the details until the very end. A large part of the fun of the story is figuring out what is going on, as the plot unfolds, and seeing the pieces fall into place.
The main risk with approach 2 is that the character's actions won't make sense without their motivation. The audience needs to understand the actions of the character at least a little to stay with the plot (unless you're writing an avant-garde play). In inception, this is solved by disclosing Cob's motivation in different levels (which is apposite given the theme of the film): we learn very quickly that he needs to complete a complex heist, and that he has some important reason to take a big risk. This is enough information to follow along at the start. Then bit by bit, we learn more about what exactly he wants, how he got into this situation and why he makes certain odd choices (such as not designing the dream world himself). The revelation of the entire backstory is explicitly saved up as the big reveal of the film (taking precedence over the result of the heist itself).
Another common trick that Inception uses is to introduce a POV character other than the traumatized individual (in this case Ariadne). This gives the audience a character they can sympathize with while the protagonist is still ambiguous. Nick Carraway in the Great Gatsby is another example of this.
One of the main difficulties with option 1 is that the trauma isn't very meaningful until we're invested in the character. This is why it's so often massively over-the-top (home invasion, rape/murder etc) and still rings quite hollow. This is one benefit of the second approach, a much smaller trauma, that feels more realistic can be much more shocking if we already know the character. I can't think of any story that gives trauma up front and still manages to solve this problem.
In general I would say it's best to go for one of the extremes, rather than something in the middle. Once you've made your choice you know what your job is. Either you set up the stakes right away and slowly unfold the backstory step-by-step, or you set up the trauma right away, and you slowly let the consequences unfold.
As for finding the moment and the pace, you could start by breaking up your backstory into a series of reveals, and spread these throughout the story. Inceptions has its early reveals when there is a lull in the ongoing action, but the big reveal happens just as/before the heist climaxes. This is good for action driving development.
The Great Gatsby has more of a tragic structure, with the reveal in the middle, and the world unraveling slowly after. What works for you depends on the story and the genre.