Response of narrow QRS tachycardia to premature atrial extra. What is the mechanism?

A 60-year-old woman was referred for radiofrequency catheter ablation of narrow QRS tachycardia that was terminated with intravenous adenosine. Twelve-lead ECG showed no baseline preexcitation. Echocardiogram was essentially normal. The electrophysiological study showed a normal AH interval of 114 ms and HV interval of 48 ms during sinus rhythm. Anterograde study demonstrated no dual AV nodal physiology. Atrial pacing protocols easily and reproducibly induced narrow QRS tachycardia (Fig. 1A & 1B). Premature atrial extra (PAE) stimuli were delivered during the tachycardia (Fig. 2). What is the mechanism of the tachycardia?