It is standard race car practice to use a swirl pot in the cooling system. If hot spots in the engine cause localised boiling of the water, small pockets of air will form. Air in the water will reduce the heat transference properties of the cooling system. If correctly positioned & designed, the swirl pot will spin the water around inside it, making any water pockets form in the middle of the water. Percolation will then make this air rise inside the swirl pot, and exit out of the small bleed tube in the top, back to the header tank.