Final answer:
The S2 heart sound, also known as 'dub', is heard during the early phase of ventricular diastole, specifically in the isovolumetric relaxation phase when the semilunar valves close to prevent backflow of blood.
Step-by-step explanation:
The second heart sound, S2, is heard during the early phase of ventricular diastole, when the semilunar valves close. This closure occurs when the pressure in the ventricles falls below the pressure in the pulmonary artery and aorta, preventing backflow of blood into the heart. This phase is referred to as the isovolumetric relaxation phase because there is no change in the volume of blood within the ventricles since the atrioventricular valves remain closed at this point.