|Create a white matte. (FILE/NEW/COLOR MATTE).|
Put it on your timeline on a track above the clip you want to "flash".
Right-click on the white matte.
Adust the DURATION so it reads 10 frames (00;00;00;10).
Press PgUp (PAGE UP) to find the beginning of the white matte clip.
In the OPACITY control, insert a keyframe. Make sure opacity is 100%.
Press PgDown (PAGE DOWN) to find the end of the white matte clip.
Set opacity to 0%.
VIDEO-2 Color (white matte)
VIDEO-1 Your clip to be "flashed"
This will give you a "flash" effect. You can move the white matte clip around to fine-tune where you want the flash to occur.