When David cooks stir-fry, he is a big believer in “guo qi” (锅气) – as Grace Yong calls it – the “breath of the wok”. And I have never hear of this before meeting him – just like how he has never wrapped a dumpling. It turns out to be a major Cantonese cooking concept.
Basically the idea is when you stir fry, you cook over extremely high heat. The food cooks rapidly in the high heat, retains tons of moisture/tenderness and takes on a yummy flavor due to a small amount of browning. To achieve this, you need to know when you pan is hot enough.
There are tons of ways to figure this out – you can drop water into the pan and see how it sizzles, you can put your palm 1-2 inch above the pan bottom and feel the heat, or… you can buy an IR temperature sensor and measure it.
Do you need this? Probably not but it is pretty fun. I also use it when I want to season the cast iron pan or understand how temperature is distributed when searing. ^.^
For stir fry, you want the pan to be north of 400F and continue on high heat.
You can’t do this with non-stick pans!