Or you could do this:
Video Effects> Brightness and Contrast or Glow or Levels or whatever
Cut the clip where you want the flash to start.
Apply the method of flashing to the clip (I chose Brightness for this one) and set the level of brightness you want the flash to start at.
Bring the brightness or effect level or whatever the hell of the effect back down to normal or 0 or whatever where you want the flash to end.
Yeah, at least that's how I do it o.O
Eh~ Sorry man, I'm tired as hell and decided to make an attempt at being helpful-ish before going to sleep. So if you can't understand, I'd recommend never listening to me again, because sadly, I don't get much smarter o.O