Page 140 - ECG dr shamol_Slide

We know Septum is depolarized by from left to right by LBB
But here LBB is blocked sot it will be depolarized by the RBB from right to left .

 As the Vector direction toward the left that means away from V1 and toward

the V6 .
That why it cause negative( Q ) in V1 and positive wave (R) in the lead V6