on Jane Friedman site:

They’re the bogeymen of publishing. Along with prologues, adverbs, and semicolons, flashbacks may be the most vilified—and most misunderstood—of storytelling devices, ones that work only if they don’t seem like devices.

Yet flashbacks are inherently artificial. Even when we are revisiting memories in life, we rarely replay an entire scene from start to finish, chronologically and in full detail. Memory doesn’t work that way; it’s slideshows and not a movie.

But one prime reason that flashbacks are a common literary convention is that, used well, they can be an effective way to present essential information and backstory. Readers have become trained, as with so many fictional devices, to accept the artificiality of flashback provided it doesn’t interrupt their experience of the story.

And there is where the trap lies that so often derails an author’s attempt to use flashback: If not woven seamlessly into the flow of the story, a flashback can draw attention to itself, reveal the author’s hand, and pull the reader out of the fictive dream.

But you don’t have to avoid this potentially potent device as long as you follow a few key guidelines in weaving flashbacks seamlessly into your story.