Final answer:
The 'dupp' or the second heart sound, S2, is heard when the semilunar valves close, which occurs just after the T wave on an ECG during the ventricular diastole phase.
Step-by-step explanation:
The second heart sound, often described as the 'dupp' is heard during the ventricular diastole phase and corresponds to the closure of the semilunar valves in the heart. This event can be observed on an ECG where it occurs just after the T wave, representing the ventricular repolarization. When the semilunar valves close, they prevent blood from flowing back into the ventricles from the aorta and the pulmonary artery, creating the characteristic 'dup' sound of the second heart sound, S2.