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In the pilot episode of "The Americans" Elizabeth has a flashback to something that happened twenty years earlier. Seeing her reflection triggers the flashback, but a reflection seems to have no earlier reference in the plot line. The flashback makes sense once the scene is over, but that is after the fact.
Is it an accepted technique for the protagonist to have an internally logical flashback, even if the audience doesn't know the reason for the flashback?
I'd argue that most flashbacks make sense only later in a story, exspecially in visual media, so yes, it is an accepted technique.
A lot of times a flashback is used as a for of foreshadowing: it is somewhat implied that what is being showed will make sense at a later time. You can still run into trouble if the flashback is out of place (e.g., the reason doens't become clear later on, or is not a strong reason). But that's a whole other topic.
The flashback is informing the audience; both literally and metaphorically she is looking at herself, and the flashback is about her past. Yes, it makes more sense later, but by showing us the flashback the author is telling us the woman is not just who she appears to be. Not to herself, and later, not to us.
That is my take on it, others may disagree. The fact she is looking in the mirror is important.