Final answer:
The vulnerable period occurs on the downslope of the T wave during the cardiac cycle, representing ventricular repolarization on an ECG.
Step-by-step explanation:
The period on the downslope of the T wave, also known as the vulnerable period, is the phase in the cardiac cycle where the heart's ventricles are repolarizing. This phase is critical because, during this time, the ventricular myocardium is sensitive to disruptions in its electrical stability, which can lead to abnormal rhythms. The T wave of an electrocardiogram (ECG) is the component that represents the repolarization of the ventricles, and this period follows the ventricular contraction known as systole.