Every now and again, I read about how screenwriters shouldn’t ever use flashbacks in a story. They break your narrative for this and that reason and if you so choose to make use of them, then a curse on you.
However, the people saying this hardly ever explain why flashbacks don’t work and what you could do to make them work. It’s true that a flashback on its own can break narrative momentum. Why this happens is because they usually force background information down your throat at times that seems tacked on to the story rather than a natural part of it.
Now, what you can do to make flashbacks work is to use them to tell a subplot set in the past. You see this in Gone Girl where the subplot that reveals the past to make you sway on the killer being the husband is told through a series of flashbacks. In this case, the flashbacks work. You also see something similar in The Lake House, where the Keanu Reeves story set in the past builds to reveal a death that happened in the future, which we see at the start of the film. This isn’t technically your traditional flashback, but it still counts.
Thus, it could be said that flashbacks on their own hardly work, but if you use them to explore the past across an entire subplot, which more or less will collide with the main plot around act four, then flashbacks become a useful storytelling technique.