A 52-year-old woman from Bangalore presents with palpitations and syncope. On examination, heart rate is 160/min and irregular. ECG shows narrow-complex tachycardia with a rate of 160/min. Carotid massage is attempted and the arrhythmia terminates abruptly, revealing a normal sinus rhythm with a PR interval of 180 ms. Transthoracic echocardiography is normal. Which cardiac cycle phenomenon explains why carotid massage terminated this arrhythmia?