The rules don't distinguish between actions performed in the present moment and those performed in the past. When a scene is ongoing, you can invoke a flashback to roll for an action in the past that impacts your current situation. Maybe you went to a bar last night and befriended a couple of guards to make them drunk so that they call sick the next day. You then do a flashback scene, and make a Bond roll to see how that went. The GM sets a cost in momentum for you to activate the flashback.

0 momentum: An ordinary action for which you had easy opportunity.
1 momentum: A complex action or unlikely opportunity.
2 (or more) momentum: An elaborate action that involved special opportunities or contingencies.

After the momentum is consumed, a flashback action is handled just like any other action. Sometimes it will entail an action roll, because there's some danger or trouble involved. Sometimes a flashback will entail a fortune roll, because we just need to find out how well (or how much, or how long, etc.). Sometimes a flashback won't call for a roll at all and just consuming the momentum will be enough. When you are using flashbacks, you are focusing on the problems that do happen, not the problems that might happen.

Limits of flashbacks

A flashback isn't time travel. It can't "undo" something that just occurred in the present moment. If the GM described something in the scene, it is now established in the fiction. While you can't undo what has already happened, you can twist and adapt things that were left unsaid. For instance, if a maître d' is making a scene to stop you from entering at a very high end restaurant, that's perhaps because you Swayed them into doing this the night before in exchange for money to make a diversion for the rest of the group.

Recap

Flashback
Use this to handle the unpredictable.

GM sets a cost in momentum (0-2) for you to do a flashback scene.

Depending on the fiction, either

the player gets what they want.

the GM makes a fortune roll to see what happens.

the player makes an action roll to see what happens.

PreviousDetermination Roll

NextMomentum