In Italian cooking, the starchy pasta water is an essential ingredient. It adds taste, a certain thickness to a tomato based sauce and integrates the pasta with the flavour. To complete this rite of passage the best way to perfect your red pasta dish is to return the undercooked pasta to the sauce in the pan and mix by folding. Add a ladle of pasta water and bring to the boil for just a minute.  Anticipate al dente accordingly.