Transition during radiofrequency ablation of manifest preexcitation. What is the mechanism?

A 45-year-old lady was evaluated for recurrent episodes of palpitation. Her 12­lead electrocardiogram (ECG) showed manifest but incomplete pre-excitation consistent with left sided accessory pathway. There was no structural heart disease by echocardiogram. She underwent an electrophysiology (EP) study after informed consent. A narrow QRS tachycardia consistent with orthodromic AV re-entry got induced. The accessory pathway was mapped to 2 'O clock of the mitral annulus (MA). While radio frequency ablation (RFA) was being performed at that site, an interesting phenomenon was observed (Figs. 1 & 2). What is the mechanism?