Bring guanciale and 3 cups water to a boil in a small saucepan, then cook at a rapid simmer 10 minutes (5 minutes if using bacon) to render some fat. Drain, reserving 2 cups water. Blot guanciale dry.
Boil collard greens gently in a large pot filled three-quarters full of water until tender, 18 to 20 minutes. Drain, rinse with cool water, and drain again. Coarsely chop.
Cook garlic in oil in a large frying pan over medium-low heat until sizzling, 2 to 3 minutes. Add guanciale; cook 2 minutes, then transfer garlic to paper towels. Continue cooking guanciale, stirring often with a long-handled spoon (it may spatter), until crisp, 6 to 10 minutes. With a slotted spoon, transfer to paper towels.
Break arbol chile in half, add to oil in pan, and toast until puffed, 1 minute. Add greens, reserved garlic, and salt and pepper and stir until coated. Add reserved cooking liquid and cook over medium-high heat until most of liquid evaporates but greens are still juicy, 10 to 15 minutes.
Stir in guanciale. Spoon greens into a dish and scatter Calabrian chiles on top.
*Find guanciale and Calabrian chiles on amazon.com or from an Italian deli or grocer.
Note: Nutritional analysis is per serving.