STEP ONE. Only partially cook dry pasta. In a pot of abundantly salted boiling water, cook dry pasta for only half or three-quarters of the recommended time. The timing depends on the brand, shape and size; I recommend reading the back of the package instructions. Tasting pasta is the best indication of doneness, and in this case, it should be too hard to eat, but soft enough that you can taste the dough.
STEP TWO. Add the semi-cooked pasta to a pan sauce. Using a slotted spoon (for shaped pasta) or tongs (for spaghetti), remove the partially cooked pasta from the water, reserving the water for later use. Add the pasta to the pan of sauce. For example, when I cook spaghetti with clams, I saute the clams in olive oil, garlic and wine, and as they open and release their liquid, I transfer the semi-cooked pasta straight from the pot of boiling water in the pan with the clams.
STEP THREE. Adjust the thickness of the sauce by adding the reserved water. Finish cooking the pasta in the cuisson (cooking liquid). Add the reserved water to adjust the thickness of the sauce. Reduce by simmering and season as necessary, bearing in mind that the pasta water you are adding to the pan is heavily salted. If you reduce it too much, the dish can get salty.
Recipe courtesy of Scott Conant, Chef's Secrets, Chronicle Books, 2004