A 30-year-old male with electrocardiographic (ECG) evidence of manifest pre-excitation (figure 1) was referred for an electrophysiology study (EPS). Quadripolar catheters were placed in the His position (His) and right ventricular apex (RVA) and a decapolar catheter was placed in the coronary sinus (CS). Ventricular pacing (VP) with and without isoproterenol infusion revealed poor retrograde ventriculoatrial (VA) conduction in both the atrioventricular node (AVN) and the accessory pathway (AP). Single atrial extra-stimuli (AES) were introduced after a drive train at 400ms from poles CS 9,10. The following reproducible phenomenon was observed (figure 2A, 2B). What has been unveiled, and how is it explained?