We can see that at a certain cycle length, there is a conduction block in the left bundle, which produces a left bundle branch block (LBBB). When a ventricular extrasystole occurs, it reaches the AV node while it is still refractory, so it is blocked. The following beat is therefore delayed, giving the left bundle enough time to recover, and the QRS becomes narrow again. Then, when the heart rate increases, the left bundle becomes refractory once more, and the LBBB reappears

SIMPLE 😀