Final answer:
During early ventricular diastole, the AV valves are forced closed, producing the "lubb" sound.
Step-by-step explanation:
During the early phase of ventricular diastole, the AV valves are forced closed, producing the "lubb" sound. This occurs as the ventricular muscle relaxes and pressure within the ventricles drops below pressure in both the pulmonary trunk and aorta. The closure of the AV valves prevents backflow into the heart.