Like backfill, flashbacks pull your reader away from the action. But this doesn't mean you have to avoid them at all costs. Too many flashbacks and you might be telling the wrong story, but flashbacks do serve a purpose: they offer additional insight into a character and can bring the reader up to speed.
Still, we have to know about the character's present situation if we're going to care at all about their past, so maybe beginning a story with a flashback isn't the best idea.
But if you need to write a flashback into one of your scenes, know that it has to be strong enough to keep us interested. This means it should offer value to the story and add relevant depth to the character or storyline.
Be clear: we should know that it's a flashback and approximately when it took place in relation to the character and their current situation.
No flashbacks for the sake of filling space.
And make sure that appropriate "kick" (our swimming pool theory) is there to help us navigate to the other side.
Flashbacks, when done right, can offer the reader some much-needed breathing room to help process what's taking place.
Again, balance is key.
Be Brilliant!
~Katie~