My method was: each Flashback can be used a second time, when all your current ones are Used, for a lesser effect.
Strength: use this to automatically succeed an FA roll, with a roll equal to your FA score. Thus, you can remove one threat automatically, or cause the turn to be postponed.
Weakness: use this to fail a turn, but avoid any effects that you would have suffered that turn. Handy when you failed, or fellow players ganged up on you.
Both must be narrated appropriately, and after using them, change the / in the Used box to a X to show this benefit has been used.