Mantecatura is the essential Italian technique of finishing pasta in its sauce with starchy pasta cooking water, tossing or stirring vigorously to create a creamy, emulsified coating that binds sauce to pasta in a unified, glossy whole—the single most important technique in Italian pasta cookery that separates a great plate of pasta from a mediocre one. The term comes from 'mantecare' (to make creamy, to emulsify), and it describes the final 1-2 minutes of pasta preparation where the nearly-al-dente pasta is transferred directly from its cooking water into the sauce pan, along with a ladleful of the starchy cooking water, and the whole is tossed or stirred vigorously over medium heat until the sauce thickens, coats every strand or piece of pasta, and takes on a glossy, creamy sheen. The science is straightforward: pasta cooking water is rich in dissolved starch, which acts as an emulsifier and thickener when combined with the fats in the sauce (olive oil, butter, guanciale fat, cheese). The vigorous tossing creates turbulence that breaks fat into tiny droplets, which the starch molecules surround and stabilise—creating an emulsion. This is why cacio e pepe, carbonara, and aglio e olio become creamy without any cream: the starch from the pasta water emulsifies the cheese fat and olive oil into a smooth sauce. The technique applies to virtually every Italian pasta dish: the pasta should never be drained, sauced, and served—it must be finished IN the sauce with its starchy water. The mantecatura is also the technique used to finish risotto (with butter and Parmigiano) and certain braised dishes.
Transfer pasta to sauce pan before it's fully cooked. Add starchy pasta cooking water. Toss/stir vigorously over medium heat. The starch + fat = creamy emulsion. Pasta finishes cooking IN the sauce. The result is a glossy, unified, creamy coating. Never drain pasta and add sauce on top.
Use slightly less water than usual when cooking pasta—a more concentrated starch solution emulsifies better. Transfer pasta with tongs or a spider (don't drain—you want water clinging to the pasta). The sauce pan should be large enough for vigorous tossing. For cacio e pepe and similar dishes, remove from heat before adding cheese to prevent clumping. If the sauce breaks, a splash of cold pasta water and vigorous tossing can re-emulsify it.
Draining pasta and pouring sauce over it (the pasta and sauce must marry in the pan). Using too little pasta water (not enough starch to emulsify). Not tossing vigorously enough (emulsion requires agitation). Overcooking pasta before transferring (it continues cooking in the sauce). Using a pot that's too large for the water (dilutes the starch concentration).
Marcella Hazan, Essentials of Classic Italian Cooking; Serious Eats, The Food Lab