Mechanism of valvular plane. Because the cardiac apex is firmly connected to the diaphragm, the valvular plane changes its position depending on the state of contraction of the cardiac musculature.
During the phase of ejection (A, systole), the ventricular musculature contracts and thus pulls the plane "downward" towards the cardiac apex. This causes negative pressure in the atria, so that they can "suck in" more blood out of the venous system.
During the phase of filling (B, diastole), the musculature relaxes and the valvular plane moves towards the atria, which contract, the atrioventricular valves open up, and the ventricles can be filled again.