During joint diastole,

During joint diastole, the ventricles relax and the ventricular pressure falls causing the closure of semilunar valves which prevents the back-flow of blood into the ventricles. As the ventricular pressure declines further, the tricuspid and bicuspid valves are pushed open by the pressure in the atria exerted by the blood which was being emptied into them by the veins. The blood now once again moves freely to the ventricles.