Used frequently in Anime to either recap past events or provide a back story for a character, either their origin story or their off-screen training of new attack. Sometime and entire episode/chapter would be a entire flashback.
There is more than one way for a character to remember something in a flash back, all depends on their repsected mediums
Filter: This is when there is a filter around the screen.
Sean's dark past
One panel Falshback: These areused to sum up an entire character's back story within one panel with a brief narration to explains, other times they are used as foreshadowing or hinting of a character. In all cases, the details of in this is usually bit obscure or ambiguous.