To prepare pork, combine garlic, brown sugar, chile flakes, salt and pepper. Rub marinade all over pork. Cover and refrigerate for 1 hour to up to 24 hours.
Set electric pressure cooker to sauté (or use a large skillet). Add pork in batches and sear until browned all over, about 2 minutes per side. Add 3/4 cup water to pot (or to skillet to deglaze, then move to pot), cover, and set to cook for 90 minutes on high pressure. Make sure to scrape the bottom of the pot so it doesn't scorch or the pot will turn off.
While pork cooks, prepare sauce: In a small pot, warm peanut oil over medium heat. Add garlic and ginger, and sauté until fragrant, 1 to 2 minutes. Add remaining ingredients and bring to a simmer. Cook until thickened, 1 to 2 minutes. Set sauce aside. (It can be made up to 1 week ahead and stored in the refrigerator.)
Manually release steam. Let pork cool until you can handle it, then shred it into bite-size pieces. Pork can be made in advance.
While pork cools, strain liquid from bottom of pot. Pour off fat (or chill liquid, then scoop off solidified fat with a spoon). Reserve.