Shred potato into very thin slivers. While you can use a knife if you have the skill to produce long, thin matchsticks, it's difficult to get them thin enough and properly shaped. A mandoline with "teeth" for shredding, set on the thinnest slicing setting, produces perfectly squared slivers. A food processor with the grating blade also works. A handheld grater produces a ragged, less uniform shred.
Soak the shredded potatoes in water for 10 minutes, washing and scrubbing them in at least two changes of water. The starch will turn the water cloudy, so rinse until the water runs clear. You will notice that upon first soak, the shredded potatoes appear limp, but after 5-10 minutes they stiffen up. Set aside to drain as completely as possible.
In a large wok or skillet over medium heat, add enough oil to coat the bottom and sides of the pan. Heat til smoking and let cool (SKIP smoking step if not using caiziyou). Bloom the huajiao in the cold oil until it starts to sizzle. Add the chopped dried chilies. When you can smell the huajiao and chilies, add the sliced garlic and stir-fry for 10 seconds or until fragrant. Turn the heat up as high as possible and add the shredded potatoes, stir-frying 2-3 minutes.
When potatoes are soft on the outside while still slightly crisp in the middle, turn off the heat and add in the salt and MSG (optional). Quickly pour in the black vinegar around the edges of your wok or skillet to avoid getting the potatoes soggy. Toss to combine. Top with the scallions and serve immediately.