The way I've done it is to cut the clip where you want the flash and then put fade out to fade in really fast. There's probably an easier way but this gets the job done.
"pee on the zombies, because not even zombies want pee all over them" - ktwofreeskier
"Never before in my time at the bar or on the bench have I ever had to deal with somebody who voluntarily allowed himself to be buggered by a dog on the public highway. Frankly it is beyond most of our comprehension" - British Judge