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A specific type of Flashback Effect.

It happens all the time, you slip in the shower or remember something funny and whoosh! You're in a Flash Back, and everyone knows it.

This is compounded by the familiar technique of echoing whatever phrase triggered the flashback as the scene slips into the past. ("Are you okay... okay... okay...?" And then you're in the Flash Back.)

Some series don't use an echo, employing instead some other sound effect such as harp music or a bell, usually accompanied by a blurring/fading dissolve to the memory.

A typical subversion is to have too many echoes or have the character provide their own.

Flashback Echo is a different trope entirely.

Film

 * Parodied in the Waynes World movie as Wayne and Garth (in their typical low-budget way) literally walk on screen to back the plot up and work out a better ending by waving their hands across the screen and going "Doodooleeloo! Doodooleeloo!"

Live Action TV

 * Lost uses a whooshing sound to indicate flashbacks, although one can usually tell by the setting not being the island.
 * Let's not forget the "Thousand-Yard Stare of Impending Flashback."
 * As an interesting aside, the sixth season changed its "flashback" mechanic to highlight a "Sideways" world. Accordingly, the producers changed the traditional flashback noise to a noise more akin to an airplane lifting off, drawing a distinction between flashbacks and "flash-sideways".
 * Chewbacca in The Star Wars Holiday Special. It's subtle.
 * Doctor Who plays with this trope in the episode "The Unicorn and the Wasp". Even the Doctor starts to get caught in a flashback at one point.
 * Parodied in Frasier. After reminiscing about the past, the characters look dazed and introspective as harp music is playing in the background, a typical cue for a flashback scene. A moment later they snap out of it. Still hearing the music, they turn around to see that someone behind them was actually playing a harp.
 * Sabrina the Teenage Witch parodies it, Salem tells Sabrina to make the music for him to tell a story in flashback, Sabrina does just that.
 * Parodied in the modern Adams Family show. Fester says something which causes Morticia to flashback as that last word is echoed over and over. When the flashback ends, it's revealed that Fester is just saying that last word over and over.

Radio
"Nebulous: Did you say...clown?
 * Firesign Theatre's The Further Adventures of Nick Danger. "No, no, no, no, no, no! You don't understand how radio works. Now, this is my flashback -- all I have to do to return us to the present is fade my voice out like this and cue the organist!"
 * In Nebulous, the Professor's frequent flashbacks to his childhood at the circus are heralded by an echo of him saying "I'm not a clown". Lampshaded when at one point, a character asks why he was lying on the floor muttering the world "clown", and at another time when the word "cloud" sets him off instead.

Sir Ronald: No! I said "cloud"!

Nebulous: [flashback beginning] I don't want to be a clown...clown...clown...

Sir Ronald: [voice fading into the background] I said cloud!"

"Asso: I started to think back. Back to the beginning of this case...
 * The musical variation is parodied in Son of Cliche, in the first of the recurring sketches "Asso, Spanish Detective".

[sting of harp music]

Asso: I was in my office, playing my harp."

Web Comics

 * Stickman and Cube couldn't resist having a go at this one: Stickman once patiently explained to Cube how to flashback, including repeating the last word of your sentence over and over again, and the obligatory sound effect. Cube then used these techniques to flash back to Stickman teaching him how to flashback. Cue neverending loop.

Western Animation
"Professor Monkey-For-A-Head: I remember it as if it were yesterday...yesterday...yesterday...
 * Parodied in The Fairly Odd Parents in an episode where several Flashbacks occurred, each time preceded by a character providing the echo themselves: "...while he was away at summer camp...camp...camp." Further parodied when Timmy's dad gets an echo response to "summer camp" while in the flashback; he adds "Put up drapes to cancel the echo" to his to-do list.
 * In typical South Park style, in the episode "I'm A Little Bit Country", Cartman tries to avoid research on the Revolutionary War by inducing a flashback. This mostly involves getting himself hit in the head, and saying "I wonder what things were like back then. Back then. Back then..."
 * The Simpsons had Marge flashing back to Homer's words, only to reveal that Homer was whispering them to her through a cardboard tube, including the echo.
 * In another episode, different characters trigger flashbacks by actually repeating the words "think back".
 * Subverted in one Family Guy episode, Peter triggers a flashback in Lois's brother by saying "Pow! Right in the kisser", while dressed up as Ralph Kramden. Cut back to the present and Peter is actually saying the phrase over and over.
 * While most of the flashbacks in the Supertrooper duology of Galaxy Rangers avoid this, the scene where Shane is forced to become a Hunter of His Own Kind uses this effect partly to make the scene feel even more oppressive.
 * Parodied in Duckman. An episode begins with Duckman waking up tied to a table and wondering how he got there. He thinks "Think back, back, back..." and flashes back...to him waking up tied to a table just a few seconds ago. This time he thinks "Think back, back back...must think back a little farther, farther, farther...". He flashes back to waking up again. Rinse and repeat, but this time he adds "Maybe volunteering for LSD research wasn't the the best way out of the Marines" and finally succeeds in flashing back.
 * Invoked in Earthworm Jim;

Psycrow: It was' yesterday.

Professor Monkey-For-A-Head: Quiet! I am setting up a flashback! Yesterday...yesterday...yesterday..."

Web Original

 * "Odion stop being such a BABY baby baby baby..........." flashes back to the time when Odion was a baby