What is your definition of "Wok Hei" (鑊氣)? Why do people say that the dishes at Dai Pai Dongs have "Wok Hei"?

Chef Paul Lau says: For food to have "Wok Hei" it has to be stir-fried under very high temperature, the heat of the wok is key to bring out the aroma of the ingredient. The Chef's technique is critical though, because the ingredient will easily be burnt if the wok is too hot.

The essence of Dai Pai Dong food is quick and no presentation is required, also they usually have only one or two staff serving a lot of customers at the same time. Hence, the stove is always under very strong heat, and dishes are ready within a few minutes. That's why they have "Wok Hei".