Response to spontaneous and induced His refractory ventricular premature beats during long RP tachycardia. What is the mechanism?

CLINICAL PROBLEM: A 43-year-old lady with no evidence of structural heart disease underwent an electrophysiological study for the evaluation of recurrent episodes of palpitations. A narrow complex regular tachycardia was documented, which was not reverted with intravenous adenosine administration. There was no basal pre-excitation. The AH and HV intervals during sinus rhythm were 70ms and 48ms respectively. A regular narrow complex long RP tachycardia with concentric atrial activation was induced which was incessant in nature. Ventricular overdrive pacing from RV septum during tachycardia at 20 ms earlier than tachycardia cycle length showed pseudo-V-A-A-V response with VA linking on cessation of pacing. A His refractory ventricular premature beat (VPB) was delivered during the tachycardia (Figure 1). A spontaneous catheter induced VPB was noted after 2 beats. What are the observations and mechanisms?