In certain explanation, its the "Charred" impression on the food itself. Best explanation is from how Charcoal Hokkien Mee is prepped.

First is the wokhei actually comes from the fragrant and ash from burning charcoal, as well their steady and powerful flames.

No wokhei usually comes from food that was cooked half heartedly, for example, an egg fried rice or Yongchow Fried rice.

Without wokhei, it means the wok is heated before splashed with cold oil and then egg masuk, which during the process, the flame intensity reduces to avoid charring then masuk rice and charsiew veggie etc, then pelempang to loosen, then tossing, then flavoring, up serving on plate. In between this, perhaps water is added to ensure the rice is hotter by the steaming vapor, and to avoid the rice become too dry from the heat.

With wokhei, it means the wok is heated to maximus before cold oil bath and then egg masuk, all without removing the heat. The process is about the same, but without the api besar api kecil cooking in between but more of penampar and pelempang until the fried rice is done. End product is one cook in stable constant heat until removal, and the former is handled carefully to avoid char.

If this process done on hokkien mee, the wokhei is heavier because when water masuk, all the flavor from the char masuk air, making it brothy and saucy, which thicken the smoky flavor.

Can. What you need is to prep your stuff well before cooking, and avoid api power masak 30 second, stop aside potong potong. That way, what hei also die liao.

The most important part to "Hei" you wok for it to start heat that shit to burning before slapping some cold oil on it then start cooking. (Try to avoid disaster by accidentally pour oil while your hand is wet or something. careful cooking is best to get that wokhei.

If all fail, get a skillet. Skillet is best to char the hell out of the dish you want to do, without burning the thing to flavordeath.