Step 1: Build the Sweet and Sour Sauce - Pour the brown sugar, ketchup, soy sauce, cornstarch, and reserved pineapple juice into
your crockpot. Stir everything together until the cornstarch is worked into the liquid and you do not see dry pockets floating around. This first stir matters because cornstarch thickens best when it is evenly distributed. At this stage the sauce will look fairly thin, and that is exactly what you want. As the slow cooker heats up, the cornstarch begins doing its job and the sauce gradually turns glossy and thick enough to coat the meatballs.