Discordant responses to his refractory premature ventricular beats (PVBs) during a regular narrow QRS tachycardia. What is the mechanism.

A 32-year-old lady was evaluated for recurrent episodes of palpitation. During one of the palpitation episodes a regular narrow QRS tachycardia was documented, and it got terminated with the administration of IV adenosine. The baseline 12 lead electrocardiogram (ECG) did not show any manifest preexcitation. There was no evidence of structural heart disease by echocardiogram. Patient underwent an electrophysiology (EP) study after informed consent. Quadripolar catheters were placed at the His region and right ventricular (RV) apex. A decapolar catheter was placed in the coronary sinus (CS) with CS 9, 10 dipoles at CS OS region and CS 1, 2 dipoles at CS distal region. A mapping & ablation catheter was positioned at right atrial (RA) appendage. Baseline atrial and ventricular pacing protocols could not be performed as both atrial and ventricular pacing were easily inducing a regular narrow QRS tachycardia. His refractory premature ventricular beats [PVBs] were delivered from RVRV apex and left ventricular [LV] free wall. Discordant responses were obtained. What is the mechanism?