, a flash is a move where all the balls are in the air at the same time.
Thus a three ball flash would go something like 33333555(wait)3333. You can throw a flash with any number of balls, although I'd be surprised to see it done with any more than five.
The simple trick to a flash is to throw all the balls to the same height. If you can do this, they will land in the proper 333 rhythm. Don't panic.
Now, since your hands are free during the flash, you should do something to show off this fact. (You want the audience to see what you're doing.) By far the most popular thing to do is to spin round before catching the balls; known as a "flash pirouette". But don't just learn this one variation - be creative!
NB: A 'flash' is different from 'flashing'. 'Flashing 7 balls' means that you are throwing the first few throws of the 7 ball cascade consistently, not that you are doing a flash with seven balls!