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A 25 year old man underwent electrophysiology study for recurrent symptomatic paroxysmal palpitations. There were no documented episodes of supraventricular tachycardia on surface ECG. Delta waves were absent on baseline ECG. A decapolar catheter was placed in the coronary sinus with the distal and proximal pair of electrodes configured as CS 1-2 and CS 9-10 respectively. Quadripolar catheters were positioned in the high right atrium, His-bundle region and right ventricular apex. During straight atrial pacing with a drive-cycle length of 340ms, the following arrhythmia was induced (Figure 1). The corresponding electrogram is shown in Figure 2. What is the mechanism?