The phase of rapid repolarization (Phase 3) occurs when the gradual accumulation of potassium ions on the outside of the membrane reaches a critical value which causes a dramatic increase in the conductance of the potassium ions across the cell membrane. Potassium ions then flow rapidly across the cell membrane from the inside to the outside of the cell and the cells rapidly repolarize restoring the transmembrane potential to its resting level. This occurs in a sequentional manner and corresponds to the T wave on the electrocardiogram.
The onset of the T wave corresponds to the onsetof rapid repolarization (i.e. the end of the plateau phase); the peak of the T wave ocurs when the firsts cells in the sequence of repolarization become fully repolarized and the end of the T wave occurs when the last cells in the sequence of repolarization beome fully repolarized.