You go to the replay option and get to the part where you want to rejoin the race from and then select the flashback (on Xbox it's the blue cross). It's pretty limited though as the flashback will only take you back around 5 seconds or so and therefore unless you are really quick to go to the replay after whatever incident you want to recitify you will miss your opportunity. I hope the replay options in F1 2011 is a lot better!!