This is, without a doubt, one of the coolest tricks of the professional chef. And, let's be honest, we have all tried it at some point with different—sometimes disastrous—results. The guys at Serious Eats explain how to do it properly.
Your meal doesn't get any better when you flip it in a pan, but it's a very efficient way to mix and move ingredients.
What to toss
Small mixed ingredients when stir-frying and sautéing to get them to quickly combine and move around the pan. I also like to toss foods like pasta in their sauce, and starchy dishes like risotto, though this can sometimes be a sloppier affair, so try that with caution (and an apron).
Now, I know a lot of people like the idea of flipping pancakes, frittatas, and omelets in the frying pan. Such tosses, executed successfully, are pretty damn impressive. But I don't recommend it. Because each of these foods tends to be large relative to the size of the pan, they are much more likely to partially or completely miss their target on the way down.
How to toss
Tossing should only be attempted in cookware with curved, sloping sides, such as frying pans and woks. The mechanics of food tossing is kind of like ski jumping.
This is the starting position, with the food in the pan. Think of the food like the ski jumper before the jump.
First, it's always a good idea to give the food a quick stir with a spoon, spatula, or tongs, just to make sure everything is sliding well in the pan and not stuck. Once you're sure the food is free and clear to jump, give it a downward thrust towards the far slope of the pan.
As the food is beginning to launch out of the pan, you want to lift the front edge of the pan while pulling back ever so slightly, to redirect the food's trajectory so that it shoots up vertically, or—even better—backwards and slightly towards you.
With the food airborne and, we hope, above the pan, you next need to bring the pan slightly forward again to catch all the food as it comes down.
Execute it well and all of the food will end up right where it started and not on the floor. Note that once you get the hang of this motion, you can loop it so that the forward movement in this last step of catching the food simultaneously functions as the forward (and downward) movement of the next launch.
Here is a video that shows how the movement should be done: