The character you play in the game is native to the fiction you are creating, but you as a player are not. It is safe to assume characters are competent to deal with things in the fiction in ways you may not have thought of. This includes preparations for missions and adventures organised before they are needed.
Flashbacks are a way to cover competency of character and forego any need to plan ahead of the action. You are cutting to the action and retro-actively doing any Moves or fiction required in a flashback.
There is a Stress cost for Flashbacks based on how much preplanning would have been required.
After the stress cost is paid, any Moves used in the flashback are rolled for as normal. Note that if you spent 3 or more stress through flashbacks, you will now be Exhausted.
A flashback isn't time travel. An untethered flashback is.
There are multiple methods in the universe of Powered by the Paradox to use time travel, have it caused, or just retroactively infer to the universe that it occured. You may undo something that just occured in the present moment, if you sufficiently explain how it happens in the fiction. This comes at a high cost as you are fiddling with your own personal timeline.