A 58-year-old man from Delhi presents to the emergency department with palpitations and syncope. ECG shows a heart rate of 280 bpm with a regular rhythm and narrow QRS complexes. An intravenous bolus of adenosine is administered, which terminates the arrhythmia. The patient's baseline ECG reveals a short PR interval and a delta wave. During the tachycardia, which phase of the cardiac action potential in the accessory pathway was responsible for the rapid conduction that bypassed the AV nodal delay?