Second hea sound (S2) occurs during:

As the ventricular diastole stas, ventricular pressure begins to fall. Blood ejected out during systole stas flowing back toward ventricles, due to the falling ventricular pressure. This sudden change in the direction of blood marks the phase of protodiastole. Closure of semilunar valves stops this regurgitation of blood toward ventricles. This closure of semilunar valves produces S2. Thus, S2 indicates end of the protodiastole, and hence S2 is said to occur in protodiastole. After S2, the isovolumic relaxation stas.