Use a brief, clearly marked flashback to add backstory and plant a subtle foreshadowing detail, without stalling the story's forward motion.
A flashback is a short cut back to an earlier moment that explains something about the present story -- why this character is the one on watch duty tonight, or what happened the last time a false alarm was rung. A good flashback is brief and clearly marked, with a transition like "Two winters ago..." or a clear paragraph break, so the reader never gets lost about which moment is "now."
Foreshadowing is a small, natural-feeling detail planted early that hints at something important later, without giving it away -- a frayed bell rope mentioned once in passing that later turns out to matter. The best foreshadowing does not feel like a clue when first read, only in hindsight.
Mastery looks like: They insert one clearly marked, brief flashback and one subtle foreshadowing detail into their draft, without losing the reader or stalling the story.
Common stumbles: Writing a flashback so long it stalls the story's momentum; making foreshadowing so obvious it feels like a spoiler rather than a satisfying later payoff.