Why save a cup of cloudy pasta water?

As pasta boils, it sheds starch into the water. That cloudy liquid is not dirty water — it's a free thickener that's also pre-seasoned, because you salted it like the sea.

A sauce is oil-based (olive oil, butter, rendered fat) and water-based liquids don't naturally mix with oil. Starch is the peacemaker: whisked in with some vigorous tossing, starchy water lets fat and liquid blend into one glossy sauce that clings to the noodles instead of pooling under them. That's emulsification, and it's the same trick behind a pan sauce and a vinaigrette.

This is also why pasta finishes cooking IN the sauce for its last two minutes: it drinks in flavored liquid instead of plain water, and its surface starch thickens the sauce as it goes. The pasta and sauce stop being two things and become one thing.

The move: just before draining, dip a mug in the pot. You won't use all of it. But when a sauce looks greasy or too tight, a splash of pasta water plus hard tossing fixes it like magic.