Case: A 41-year-old man with no prior cardiac history and a history of regular narrow QRS tachycardia, presenting with frequent paroxysmal palpitations, was referred to our institution. The tachycardia was adenosine-sensitive, leading to the acute termination of the tachycardia. The baseline electrocardiogram was unremarkable. During an electrophysiology study (EPS), supraventricular tachycardia (SVT) was easily induced by ventricular extrastimuli. Figure [1](#fig-cap-0003) shows a single ventricular extrastimulus applied from the right ventricular apex (RVa) during the tachycardia, and this finding was reproducible. Questions: What is the mechanism of the observed response? What are the clinical implications?