Spaghetti tossed in homemade basil pesto sauce served with black olives. t will thin out the pesto so it coats everything nicely and creates a glossy pesto sauce that coats every bit of pasta. The starch in the water emulsifies with the pesto, which simply means the fat in the pesto + starch in the water thickens. Just like when you shake up salad dressings. It’s the proper way to make pastas, a technique used in every Italian household and restaurants all over the world.