Final answer:
The diastasis phase is part of the ventricular diastole in the cardiac cycle, characterized by relaxation of the ventricles with no change in volume due to closed valves until pressure in the ventricles drops and allows atrial blood to flow in.
Step-by-step explanation:
The diastasis phase refers to a part of the cardiac cycle, specifically during the period of the heart's relaxation known as ventricular diastole. In this phase, the ventricles are in a state of relaxation following the repolarization represented by the T wave of the ECG. The diastasis phase occurs after the early phase of ventricular diastole - the isovolumic relaxation phase, where there is no volume change in the ventricles due to the closure of the semilunar valves and the atrioventricular valves. This results in blood from the atria waiting to fill the ventricles once the pressure within them drops sufficiently and the atrioventricular valves open.