When a flashback is used, it is gone - it can never be used again. It would be nice to be able to call on an already defined flashback for a benefit, but without upsetting the game's balance too much.
Once per planet, you may use a Flashback that you have already Used. One you have done so, cross out that Flashbback - it can't be used again. You must describe how that Flashback applies to this scene.
The benefit gained depends on whether it is a Strength or Weakness.
You can cancel all actions in a turn, starting from the initiative point you choose.
So, say you fail with a roll of 10. Trooper Haldeman gets 8, and the aliens get 6. You are going to be injured and Haldeman is going to score kills. So, you could set your initiative to 10, and cancel all actions. Or, if you are felling kind, you could set the initiative to 7, letting Haldeman have his kills, and cancelling the actions of the aliens and any other troopers after that.
You cancel your own action, but no attack against you has any effect either.
So, when the aliens hit you, you might trigger Weakness: Scared of Spiders, and have your character jump around in panic for a round, and his erratic movement makes that attack miss.