A 72-year-old female with frequent palpitation was referred for radiofrequency ablation. The baseline 12-lead electrocardiogram and echocardiography results were normal. At baseline, the atrio-His (AH) and His-ventricular (HV) intervals were 90 and 41 ms, respectively. Dual atrioventricular (AV) nodal physiology or ventriculoatrial (VA) conduction was not observed during programmed atrial and ventricular stimulation. After isoproterenol infusion, VA conduction became decremental and concentric, with the earliest atrial activation seen at the His bundle (HB) region during ventricular pacing. A supraventricular tachycardia with a long RP interval (SVT) was induced by atrial extra-stimulation, without any jump-up in the AH interval. During the SVT, the AH and HV intervals were 180 and 180 ms, respectively, and the earliest atrial activation was recorded in the HB region (Figure 1A). During the SVT, transient 2:1AV conduction was observed (Figure 1B). Ventricular overdrive pacing at a pacing cycle length (CL) of 360 ms was performed during the SVT with a CL of 390ms (Figures 2A and B). Based on these observations, what is the mechanism of this tachycardia?