A slow cooker works by transferring the heat it creates in the base up toward the bottom of the crock. This heat then spreads to the sides of the crock and cooks the food inside. For most cookers, you would choose the low setting for meats such as a roast or pork shoulder. This setting puts out less heat, but the heat is sufficient to safely cook the food at the right temperature. High setting is usually recommended for dips and sauces that would cook for only an hour or two.
As the food cooks, a vacuum seal is created between the lid and the vessel, trapping all the steam. This liquid slowly drips back into the vessel, constantly basting the contents inside. It’s no wonder why foods come out so moist and juicy.