This answer talks about the amount of heat required for cooking fried rice which tastes similar to restaurants.
I want to understand the science behind it. Please explain.
It's an issue of thermodynamics.
When you're cooking food, the food cools itself off through evaporative cooling and the energy being used to cause chemical changes in the food (eg, caramelizing sugars).
If you have too much food in the pan, the balance is overwhelmed by evaporative cooling, and thus you can only get to the boiling point of water.
To change the equation, you need to do one of the following:
You'll often see advice for #3 -- such as patting dry steak or chicken before grilling it, as without it, you won't get good browning.
You can't do that when you're dealing with sauces. You can try cooking less, but with sauces you cause more problems -- if the pan size is the same, the area for evaporation is the same, so you don't really improve the balance.
With a sufficiently sized burner, you can actually heat sauces above the boiling point, as you're putting in energy faster than evaporation can cool it. This which will change the chemical reactions that occur, thus the resulting chemical compounds and the resulting flavor of the food.
Basically there's a specific chinese style of cooking that requires extremely large amounts of heat to get a specific mix of textures and flavours. By keeping the amount of heat high and constant, food is cooked quickly, and with a certain sort of flavour - referred to somewhat poetically as "wok hei"
Its fairly specific to chinese cooking, and something more likely to be found eating out, unless you have the right kind of high heat stove with roaring flames coming out like a rocket motor.
It's not just the amount of heat. The heat component IS really important, but that funny taste that you only get from chinese takeout fried rice, really has to do with the fire.
The smoke from the fire, even completely combusted fire envelopes the wok and the food within it, the food in that wok is absorbing the smoke and that's what imparts that Chinese takeout wok flavor. It's most noticeable in fried rice because fried rice is delicately flavored which allows you to taste it almost directly as a prominent feature/flavor of the dish. This is what is called Wok Hei and it only happens when you use fire.