I have several flashbacks planned that introduce the characters to a backstory that is too complex for a typical plot dump. The problem is that the flashbacks are triggered in the context of the story a la PTSD (post traumatic stress disorder) rather than being organized as independent play sessions, and in the one and only time I've tried this it took an incredibly long time for the player to catch on to what was happening.
I'm reluctant to explicitly clue the player in with a "Hey, it's time for a flashback!" because they're meant to be immersive, disturbing experiences and, well, PTSD doesn't exactly set friendly little boundaries around its episodes.
Is there a good way to introduce a flashback effect that makes sense in a dark, realistic period piece? Or should I just throw the character into it and hope that the confusion adds to the effect?