Add in the nappa cabbage, stir fry 2 minutes until soft.
Toss in Chinese vinegar, salt and sugar, stir fry another 1-2 minutes to serve.
Notes
Use medium-low heat to avoid burning the ingredients. When first add nappa cabbage into the pan, they probably will fill the entire pan, after cooking for a minute or two, they will turned soft and start to shrink in size.